Overcoming seed quality problems in the ginger industry

Transcription

Overcoming seed
quality problems in
the ginger industry
Marcelle Stirling
Biological Crop Protection
Pty Ltd
Project Number: VG98108
VG98108
This report is published by Horticulture Australia Ltd to
pass on information concerning horticultural research
and development undertaken for the vegetable
industry.
The research contained in this report was funded by
Horticulture Australia Ltd with the financial support of
the Australian Ginger Growers Association.
All expressions of opinion are not to be regarded as
expressing the opinion of Horticulture Australia Ltd or
any authority of the Australian Government.
The Company and the Australian Government accept
no responsibility for any of the opinions or the
accuracy of the information contained in this report
and readers should rely upon their own enquiries in
making decisions concerning their own interests.
ISBN 0 7341 0328 X
Published and distributed by:
Horticultural Australia Ltd
Level 1
50 Carrington
Sydney NSW
Telephone:
Fax:
E-Mail:
Street
2000
(02) 8295 2300
(02) 8295 2399
[email protected]
© Copyright 2001
Horticulture Australia
FINAL REPORT OF VG98108
(COMPLETION DATE: June 2001)
OVERCOMING SEED QUALITY PROBLEMS
IN THE GINGER INDUSTRY
Marcelle Stirling
Biological Crop Protection Pty. Ltd.
HA Project Number:
VG98108
Principal Investigator:
Dr Marcelle Stirling
Biological Crop Protection Pty. Ltd.
3601 Moggill Road
Moggill QLD 4070
Telephone 07 3202 7419
Fax: 07 3202 8033
Email: [email protected]
Purpose of this report:
Ginger is propagated from portions of rhizome that are known in the ginger industry as 'seed' pieces.
About five tonnes of seed pieces are planted per hectare, and this planting rate should be sufficient to
establish a dense, even crop of ginger. However, poor crop establishment has become an increasing
problem in the Queensland ginger industry in recent years. In 1997 and 1998, for example, some growers
experienced significant losses because many seed pieces rotted in the ground. Such losses affect the
financial returns of individual growers and have a major impact on the quantity of ginger available for
processing.
The main objectives of this project were to determine the causes of poor emergence and establish control
measures to overcome the problem. This report presents results of surveys for diseases in planting material
and describes experiments that improve our understanding of the etiology of these diseases. Fusarium
oxysporum f. sp. zingiber! was found to be the primary cause of poor seed emergence and the report
concludes by making recommendations on how this pathogen can be kept under control.
Funding sources and acknowledgements:
Voluntary contributions fortius project were received from the Australian Ginger Growers' Association,
Buderim Ginger Limited and the fresh market ginger growers of Queensland. Len Palmer (Buderim
Ginger Ltd.,) is acknowledged for his help with data collection and establishment and harvest of field trials.
Individual growers provided trial sites and ginger seed pieces for the trials. Ken Pegg (Queensland
Horticulture Institute) provided useful information on the disease situation in the ginger industry during the
1960s and 1970s and Graham Stirling (Biological Crop Protection Pty. Ltd.) helped with planning and field
experimentation.
Disclaimer:
Any recommendations contained in this publication do not necessarily represent the current policy of
Horticulture Australia. No person should act on the basis of this publication, whether as to matters of fact
or opinion or other content, without first obtaining specific, independent professional advice in respect of
matters set out in this publication.
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TABLE OF CONTENTS
1. INDUSTRY SUMMARY
2. TECHNICAL SUMMARY
3. INTRODUCTION
4. LITERATURE REVIEW
Diseases of ginger
Control of diseases affecting ginger
5. CAUSES AND CONTROL
Defining the causes of poor emergence
Surveys for disease in ginger
Contribution of Foz in soil to poor seed emergence
Pathogenicity tests with fungi and bacteria
Identification of bacteria and fungi
Experiments with biocides
Effect of temperature and moisture on ginger in storage
Possible improved control of Foz
Effect of acibenzolar-S-methyl
6. VEGETATIVE COMPATIBILITY GROUPS IN FOZ
7. SUPPRESSION OF FUSARIUMJN GINGER-GROWING SOILS
8. CONCLUSIONS
The causes of poor emergence of ginger
Reasons that seed germination problems are increasing
9. RECOMMENDATIONS
Modifications to current seed production/preparation practices
Introduce a clean seed scheme based on tissue-cultured ginger
Extension material
10. FURTHER RESEARCH
11. LITERATURE CITED
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1. INDUSTRY SUMMARY
Crop establishment problems in the Queensland ginger industry have increased in recent years, with some
growers losing between 20 and 90% of plants in some fields due to seed emergence problems. Surveys
done during the 1998 and 1999 planting seasons showed that the most common pathogens isolated from
diseased, discoloured or rotting seed pieces were Fusarium oxysporum f. sp. zingiberi, a well recognized
fungal pathogen of ginger, and two bacteria, Erwinia chrysanthemi and Enterobacter sp. All three
organisms were found on most farms and Fusarium and Erwinia often occurred together in the same
diseased seed piece.
When the above organisms were tested for pathogenicity in sterilized soil in the glasshouse, most isolates
of Fusarium and Erwinia were capable of rotting ginger seed pieces, but Enterobacter had no effect.
Fusarium caused a typical brown discolouration of the rhizome accompanied by some shrivelling, and most
infected seed pieces did not produce any shoots. Erwinia produced a soft, mushy rot with a characteristic
strong, offensive odour. The latter disease was most severe in wet soils, at high soil temperatures or in seed
pieces that had been subjected to mechanical damage.
Results of glasshouse experiments suggested that Fusarium is the primary cause of seed piece rot. Erwinia
is often associated with Fusarium, but it only causes disease under certain environmental conditions and is
most damaging when ginger is damaged in some way. Thus mechanical damage during harvesting and
seed preparation operations or injury due to infection by Fusarium or root-knot nematode will exacerbate
problems caused by Erwinia.
In addition to pathogenicity tests in the glasshouse, various chemical treatments were applied to seed pieces
before they were planted in the field. The results of these experiments also demonstrated that Fusarium
was the primary cause of seed emergence problems. Carbendazim and benomyl, two fungicides that are
currently recommended for control of Fusarium, gave the most consistent reduction in seed piece rot. The
efficacy of the two compounds varied between experiments, suggesting that the level of disease in different
batches of seed varied considerably. Copper-based fungicides were also effective in some experiments,
possibly because they are anti-bacterial as well as fungicidal.
Taxonomic observations of more than 20 isolates of Fusarium from throughout the ginger industry showed
that the fungus is relatively homogeneous at a genetic level. Since planting material is frequently
exchanged amongst farms, it is likely that a single strain of Fusarium has been introduced to most farms
over a period of years. Both ginger cultivars used in the industry (Canton and Queensland) were
susceptible to the fungus in glasshouse tests. However, Canton ginger has larger knobs than Queensland
ginger and is therefore more prone to damage during harvesting and washing operations. Since damaged
surfaces are vulnerable to infection from Fusarium spores during seed preparation and storage, buds of
Canton ginger are more likely to become infected and fail to produce shoots. The fact that seed pieces of
Canton have fewer buds than Queensland ginger means that Canton can sustain fewer bud infections before
emergence is affected. Queensland ginger often emerges well but the shoots wither and die later in the
season (i.e. from April onwards).
Experimental results showed conclusively that the pathogens causing poor emergence were primarily
associated with seed rather than soil. When seed pieces from eight farms were planted into soil that was
heated to eliminate pathogens, disease levels were not reduced. Also, seed pieces that appeared healthy at
the time of planting were often contaminated. For example, rhizomes from a ginger field were cut into
seed pieces and apparently healthy pieces were planted individually in pasteurised soil in pots. More than
75% of these seed pieces subsequently rotted due to Fusarium.
Evidence collected during this study suggested that growers can minimise seed emergence problems by
improving seed production and preparation procedures. Issues that must be addressed include:
• Appropriate field selection so that ginger to be used for seed is grown in land that has either never
produced ginger or has low levels of Fusarium.
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•
•
•
Inspection of seed patchesfromMarch to June and removal of rhizomes on plants with dead or yellow
shoots. This will reduce the amount of Fusarium-mfected ginger brought into the seed preparation
area later in the season.
Good control of root-knot nematode in seed patches, thus minimizing the number of entry points for
Fusarium.
Attention to detail with regard to washing, cutting, fungicide dipping and storage of seed.
A clean seed scheme based on tissue-cultured ginger is another option that could be used to overcome
emergence problems in the medium to long term. Small rhizomes produced from tissue-cultured ginger
can be planted into soil that has never grown ginger to produce a mother planting of disease-free material.
Provided strict hygiene is employed to ensure that Fusarium is not introduced on machinery or by other
means, such a planting can then be expanded to provide a continuing supply of clean ginger seed.
One other control measure found worthy of further investigation is the use of a chemical that activates the
plant's defense mechanisms to reduce the level of Fusarium infection in rhizomes to be used for seed. In
one pot experiment with a small number of seed pieces, plants sprayed with Bion (acibenzolar-S-methyl)
and then inoculated with Fusarium, did not become infected with the pathogen. If these results could be
confirmed in thefield,Bion may become a useful adjunct to the control measures listed above.
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2. TECHNICAL SUMMARY
Over the last few years, but particularly during the 1997-planting season, some Queensland ginger growers
experienced crop establishment problems. Seed pieces either rotted after they were planted or young plants
grew poorly, became yellow and eventually died. Losses of 20-30% were common, but more than 90%
mortality was reported in some batches of seed. The problem was particularly severe in Canton ginger, a
cultivar that has increased in popularity in recent years.
This summary describes the results of work done to determine the causes of poor emergence and provides
recommendations on solutions to the problem.
Disease surveys and isolation of pathogens.
Surveys of ginger plantings to be used for seed in 1998 and 1999 showed that Fusarium oxysporum f. sp.
zingiberi (Foz) was widely distributed. However, the frequency of isolation of Foz varied between farms
and between different fields on any one farm. Foz was also the main organism isolated from diseased
ginger seed pieces collected from 19 fields on eight farms soon after planting in October-November 1998.
On most farms, less than 20% of seed pieces failed to produce shoots, but losses on a few farms were
greater than 50%.
Several other fungi were also isolated from diseased or discoloured rhizomes, including Pythium spp.,
Rhizoctonia, Sclerotium rolfsii, Geotrichum, and two other Fusarium spp. The bacterium Erwinia
chrysanthemi was present on some farms and an Enterobacter sp. was isolated from all farms. In many
instances, Foz and Erwinia occurred together.
Pathogenicity of fungi and bacteria.
All isolates identified as Foz rotted seed pieces in pathogenicity tests, whereas none of the other Fusarium
spp. were pathogenic to ginger.
The Pythium isolates were not pathogenic and it seems highly unlikely that this fungus is involved in poor
emergence. The isolates tested were obtained from rhizomes in seed patches but Pythium was never
isolated from newly-planted seed pieces or seed pieces in storage.
Sclerotium rolfsii was isolated from ginger rhizomes and caused rotting of seed pieces in pathogenicity
tests. However, observations in the field suggested that it is a minor contributor to disease in seed pieces,
as its mycelium is distinctive and it was never observed in rotting seed pieces.
Geotrichum sp. was isolated from rotting ginger but was not pathogenic in two tests, even when ginger seed
pieces were severely injured.
E. chrysanthemi was relatively common but was not isolated from all farms. Pathogenicity experiments
showed that the bacterium can cause disease in ginger seed pieces, but disease was most severe in wet soils,
at high soil temperatures or in rhizomes that had been subjected to mechanical damage. In diseased seed
pieces collected from surveys and from biocide trails, E chrysanthemi was nearly always associated with
Foz. It is therefore likely that ginger is vulnerable to colonisation by E. chrysanthemi after it is damaged in
some way. Injury can occur from mechanical damage during harvesting and seed preparation, or following
infection by Foz or root-knot nematodes (Meloidogyne spp.). Experimental evidence showed that when E.
chrysanthemi was applied to seed pieces with or without Foz, seed piece rot was more severe in the
presence of Foz.
Although Enterobacter sp. was commonly isolated from ginger infected with Foz and/or E chrysanthemi,
its pathogenicity was never established in several experiments. This bacterium appears to be a common
saprophyte in soil and is probably a secondary invader.
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The source of Foz associated with seed piece rot
Rhizomes from a ginger field were cut into seed pieces and apparently healthy seed pieces were planted
individually in pasteurised soil in pots. More than 75% of these seed pieces subsequently rotted due to Foz.
This demonstrates that apparently healthy seed pieces at the time of planting can be contaminated with Foz.
The pathogen is probably present as a surface contaminant (as mycelium or conidia) and as microconidia in
vascular strands.
Soil and seed pieces were collected from 8 farms with different histories of ginger cropping. When seed
pieces were planted in heat-treated or untreated soil, levels of disease were similar. This suggests that the
pathogens causing poor emergence were present in seed rather than soil. Foz was the primary cause of
most of the rotting that was observed.
Biocide experiments
Biocides applied to seed pieces prior to planting were used as tools to help establish the causal agent/s of
poor emergence. Experiments were done at six field sites and in the glasshouse and the results confirmed
that Foz was the primary cause of the problem. Two fungicides (carbendazim and benomyl) currently
recommended for control of Foz on ginger gave the most consistent reduction in seed piece rot. The
efficacy of the two chemicals varied between experiments, confirming that there was great variation in the
level of disease in different batches of seed. These chemicals effectively reduced surface contamination of
ginger by Foz, but were probably less effective once the pathogen had penetrated rhizome tissue.
Copper-based fungicides were effective in some experiments. Since copper is anti-bacterial as well as
fungicidal, it may have acted against Erwinia, as the bacterium was present on some farms. The consistent
response to carbendazim and benomyl suggests that Foz was the primary pathogen but the response to
copper does not rule out the possibility that E. chrysanthemi was also involved at some sites, possibly by
colonising damaged or debilitated ginger.
Metalaxyl did not reduce disease, confirming the results of pathogenicity experiments that suggested
Pythium was not involved in the disease.
The susceptibility of two ginger cultivars
Experiments were carried out to confirm grower's observations that the problems of poor emergence were
in some way related to the change from cv. Queensland to cv. Canton that had occurred in recent years.
The results showed that both cultivars were equally susceptible to Foz, as disease levels were similar in the
two cultivars four months after inoculation with the fungus. However, Canton ginger has larger knobs than
Queensland ginger and is therefore more prone to damage during harvesting and washing operations. Since
damaged surfaces are vulnerable to infection from Foz spores during seed preparation and storage, buds of
cv. Canton are more likely to become infected and fail to produce shoots. The fact that seed pieces of
Canton have fewer buds than cv. Queensland also means that Canton can sustain fewer bud infections
before emergence is affected. Queensland ginger often emerges well but the shoots wither and die later in
the season (i.e. from April onwards).
Vegetative compatibility groups of Fusarium oxysporum f. sp. zingiberi (Foz)
To examine the possibility that emergence problems were due to the introduction or development of a new
strain of Foz, 22 isolates collected from nine farms were assessed for vegetative compatibility. All isolates
were grouped in VCG 0461, suggesting that the population of Foz within the Queensland ginger industry is
relatively homogeneous at a genetic level. Since planting material is frequently exchanged amongst farms,
it is quite possible that a single strain of Foz has been introduced to most farms over a period of years. The
first introduction of Foz to Queensland is likely to have occurred in the 1930's and the pathogen has
probably persisted since that time. A subsequent introduction occurred in 1954 via infected rhizomes from
China but this pathogen was apparently eliminated soon after it was introduced.
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Suppression of Fusarium in ginger growing soils
One possible explanation for farm to farm variation in the levels of damage due to Fusarium is that some
soils are more suppressive to Fusarium than others. This hypothesis was tested by inoculating various soils
with a closely related pathogen {Fusarium oxysporum f.sp. vasinfectum) and measuring disease severity on
cotton. The results showed that all ginger-growing soils were conducive to Fusarium oxysporum. Thus the
low incidence of disease due to Fusarium on some farms was probably due to long rotations (i.e. one
ginger crop every 3-4 years) and better quality control during the seed preparation process.
Improved chemical control of Fusarium
Evidence collected during this study suggested that growers could minimise emergence problems by
maintaining strict hygiene standards during the seed preparation process. Dipping seed with carbendazim
or benomyl is an essential part of this operation, but these fungicides only protect the seed piece for the first
few weeks after planting. Since all ginger-growing soils are infested with Fusarium, rhizomes often
become infected later in the season. To determine whether these late infections could be reduced,
experiments were done with Bion (acibenzolar-S-methyl), a chemical that activates the plant's defense
mechanisms. The results showed that Bion had its greatest impact when sprays were applied over a long
period (e.g. two or three sprays 10-20 days apart, interspersed with a no spray period of eight weeks). In
one pot experiment with a small number of seed pieces and low levels of disease in the untreated controls,
none of the plants sprayed with Bion developed disease. These results suggest that Bion could possibly be
useful for reducing Fusarium in seed patches, provided it is used in conjunction with other control
measures. However, the timing of Bion sprays appears to be crucial in terms of plant age and the amount
of foliage that is necessary to absorb the chemical. Thus further work is required to confirm these results
and extend them to the field.
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3. INTRODUCTION
Ginger (Zingiber officinale Roscoe) has been cultivated in Queensland since the late 1920's. Initially a
small area was established in the Buderim area but commercial production started 20 years later when
supplies from Asia were stopped during World War n. Since then, the industry has gradually expanded
and is now mainly located in Yandina and surrounding areas. Currently, between 5000 and 6000 tonnes of
ginger are produced each year for both processing and fresh markets. In the last few decades, the ginger
industry has mainly grown a cultivar known as 'Queensland', but cv. Canton (Jumbo) has gained popularity
in recent years because it produces larger rhizomes.
Ginger is propagated from portions of the rhizome called 'seed' pieces. Ginger generally harvested in late
July, August and September each year is used to prepare seed material. The planting season begins in
August and ends in late September. Early harvest commences in late February the following year, with this
young ginger being used in confection (crystallised ginger) by Buderim Ginger Ltd. Ginger harvested over
the next 9-12 months supplies fresh market outlets but is also used for powdered ginger and oil extraction.
In the past, when a large proportion of the ginger was used in processing, most ginger growers completed
their harvest by April or May, approximately eight months after planting. However, an expanding demand
for fresh ginger has resulted in some ginger crops being left in the ground forl2-18 months.
Over the last few years, but particularly during the 1997-planting season, the Queensland ginger industry
experienced crop establishment problems. Seed pieces either rotted after planting or young plants grew
poorly, became yellow and eventually died. The problem occurred on a number of farms, with more than
90% mortality being reported in some batches of seed. The problem was particularly severe in cv. Canton.
Several soil-borne pathogens are known to cause diseases in ginger in Australia and in other gingergrowing areas of the world (e.g. India, Hawaii, Japan, South Korea and Indonesia). However, there is a
paucity of literature on the actual causes of poor establishment in most of these locations. When this
project commenced, it was uncertain whether one or more of the known ginger pathogens were the cause of
poor establishment, or whether there was a new causal agent. The work described in this report was
undertaken to identify the causes of poor emergence and to develop strategies for overcoming the problem.
4. LITERATURE REVIEW
Diseases of ginger
Several soil-borne pathogens cause disease in ginger. This review describes diseases that may affect ginger
in Queensland, discusses how they are spread and outlines current management options for these diseases.
Fusarium yellows and rhizome rot
Rhizome rot in ginger wasfirstrecorded in 1930 in Queensland, but the causal agent was not identified
until 1942 (Pegg et ctl. 1974). This disease is caused by the fungus Fusarium oxysporum f. sp. zingiberi.
Initial symptoms are yellowing and stunting of plants. As the disease progresses, the lower leaves
gradually wilt and finally the shoots die. Infected rhizomes have a brown internal rot. As the rot
progresses, the soft tissue disappears and finally all that is left is a shrivelled shell containing only fibrous
tissue. The fungus is commonly introduced in infected planting material. If conditions are conducive, the
disease may progress rapidly and the seed piece will rot in the ground. Sometimes a shoot may be
produced which will die prematurely.
Once afieldbecomes infested with the pathogen, it may persist for many years because the fungus
produces relatively resistant chlamydospores. Seed pieces free from disease will become infected once
planted in infested soil. This infection occurs through growth cracks that may develop in the neck region,
or injuries caused by mechanical damage, nematodes or soil invertebrates. Infected ginger that is harvested
and stored will continue to rot. A superficial white cottony growth may be visible on the stored ginger.
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Bacterial Wilt
The pathogen Ralstonia (Pseudomonas) solanaceamm biotypes 3 and 4 cause wilt of ginger in Queensland.
This disease was first recorded in south-eastern Queensland in 1965 but its spread was restricted (Pegg et
al. 1974). Biotype 3 causes a slow wilt and is of little significance. However, biotype 4 causes rapid
wilting and death and has been responsible for heavy losses of ginger in the past. The two diseases cannot
be distinguished in the field by leaf symptoms but can be distinguished by the rate of spread and the
incidence of disease (Persley 1994).
Infected plants wilt, lower leaves turn yellow and then wilting progresses upwards until all leaves are
affected. As the disease progresses, the stems become water-soaked and easily break away from the
rhizome. Vascular tissue becomes blackish in colour. Affected rhizomes generally become darker in
colour and develop water-soaked areas with pockets of milky exudate visible beneath. When infected
rhizomes are cut, the white milky exudate flows out on application of a little pressure to the cut end.
It is uncertain how long the bacteria survive in soil. The pathogen however, has a wide host range amongst
cultivated crops such as tomato, capsicum, potato and eggplant and numerous weed species that are
common to the ginger growing areas in Queensland. Crops such as peanut and tobacco can harbour the
bacterium but do not show any external symptoms (Pegg et al. 1974). By infecting such alternate hosts the
bacterium can survive from season to season, forming a ready source of inoculum to re-infect ginger
rhizomes once they are planted in the ground.
The most important means of spreading bacterial wilt is by planting infected seed pieces. Once diseased
rhizomes rot in the ground, the soil becomes infested and has the potential to spread bacteria to disease-free
planting material. It has been suggested that insects may transfer the bacteria during feeding. Irrigation
water and contaminated farm machinery and subsequent run-off water after heavy rain may all spread the
bacteria from diseased to healthy areas on a farm (Pegg et al. 1974).
Pythium seed piece rot
This is a disease caused by unidentified Pythium spp. (Persley 1994). Initially, a brown internal
discolouration of the rhizome occurs, followed by a soft wet rot. Rhizomes finally become hollowed out.
Pythium spp. are ubiquitous in soil. The disease becomes a problem in wet seasons when rhizomes are
harvested from wet soil, or are cut and stored under moist conditions.
Rhizome, root and basal stern rot
Caused by the fungus Pterula sp., this disease is rarely a serious problem (Pegg et al. 1974; Persley 1994).
Initially, a yellowing and drying out of stems and leaves occurs. Sunken spots occur beneath the scale
leaves of the rhizome, and this symptom is then followed by a dark brown rot of the rhizome. Roots may
also be killed. In severe cases, the fungus attacks and hollows out the base of the stem. The most obvious
symptom of the disease is a complete enveloping of the rhizome with a white fungus mycelium. The
fungus survives in the soil on undecomposed plant residues from which it sends out long white strands. In
the past, the fungus has been a serious problem when ginger was planted soon after sugarcane, as it was
able to colonise the large quantity of trash left in the soil.
Rhizome and stem rot
The fungus, Sclerotium rolfsii sometimes causes a stem and rhizome rot. The fungus is visible in the form
of white cottony threads on the surface of the rhizome. Eventually, small, round, dark brown sclerotia are
seen on the infected tissue. The fungus colonises discarded ginger left in the field after harvest. It is not
considered to be a major problem.
Big bud
This disease is caused by the tomato big bud mycoplasma (Persley 1994). Plants that are affected cease
growth and leaves become bunched at the top of the stem. The pathogen is transmitted by leaf-hoppers,
particularly Orosius argentatus, which breed on a wide range of weed hosts. The insects move on to ginger
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when weeds become scarce. Incidence of this disease is low and no control measures are used. However,
affected plants are removed and destroyed.
Bacterial soft rot
This disease is caused by Erwinia spp. These bacteria are present in most soils but problems occur mainly
in water-logged fields. The disease is characterised by a softening of the tissues accompanied by an
offensive odour. The bacteria continue to develop in tissue even after the ginger has been harvested,
seriously affecting the quality of fresh market ginger.
Root-knot nematodes
Meloidogym spp., are a widespread problem in ginger especially in late harvest crops. This pest may occur
in virgin ground, has many alternate hosts and is spread in planting material. Heavily infected plants are
stunted, have yellow leaves and swollen distorted roots. Prominent galls are not produced on the rhizomes.
In severe infections, the cortex of the rhizome becomes lumpy and cracked. At this stage, small, brown,
circular, water-soaked lesions are seen in the rhizome when the corky layer is removed. Although the
rhizomes are not completely destroyed by nematodes, the market quality of the crop can be seriously
affected. In addition, damage caused by nematodes can serve as entry points for fungal pathogens such as
Fusarium and soft rot bacteria.
Control of diseases affecting ginger
Although several diseases have been discussed above, currently the major diseases affecting the
Queensland industry are caused by F. oxysporum f. sp. zingiberi and Meloidogyne spp. The soft-rot
bacteria Erwinia spp. are a major problem on some farms, especially in years when rainfall is excessive.
Control of soil-borne diseases in ginger relies heavily on a regime of stringent management practices with
some help from chemical treatments. Basically, careful handling of rhizomes to nrniimise damage at each
step from harvest to planting is essential. Discarding of damaged and discoloured planting material is
important. For Fusarium, two protectant fungicide dips (carbendazim and benomyl) are recommended.
Since this pathogen survives in soil for many years, crop rotation should also be used to reduce inoculum
levels in soil.
Soil can be fumigated for nematodes prior to planting and a registered nematicide may also be applied after
planting. Sometimes, hot water treatment (48°C for 20 min) may help to reduce nematode populations that
are not easily visible on the seed pieces. Growing ginger in furrows of sawdust can also reduce nematode
invasion and subsequent damage. Crop rotation with non-host crops will reduce populations of root-knot
nematodes in soil.
Erwinia spp. are ubiquitous and there are no chemical treatments for this pathogen. However, ginger that is
free-from injury and is not infected with nematodes or Fusarium has a good chance of success if planted in
well-drained soils.
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5. CAUSES AND CONTROL
Defining the causes of poor emergence
Preliminary observations in ginger fields experiencing emergence problems suggested that both fungal and
bacterial pathogens may have been involved. Since it was not known which pathogens were primarily
responsible or whether the causal agents were the same on all farms, surveys were carried out of potential
seed patches and also of newly planted seed pieces on growers' properties. Extensive isolations were done
of bacteria and fungi from diseased tissue and pathogenicity tests carried out to ascertain the relative
importance of various organisms associated with diseased ginger.
When this work commenced, there was a belief within the industry that the problem of poor emergence was
mainly associated with the cv. Canton. Since most growers intended to increase their plantings of Canton,
this cultivar was used for most of this investigation.
Surveys for disease in ginger
Methods
1998 survey of diseases in the field. This survey concentrated on cv. Canton but two fields of cv.
Queensland were also assessed. Ginger in fields to be used for seed in the 1998-planting season (AugustSeptember) was assessed for disease in April-May 1998. Sections offields(several metres of row) were
randomly selected and each plant (i.e. a single much branched rhizome and numerous aerial shoots known
as pseudostems in the designated area) was examined in situ for disease symptoms. These symptoms
included shoots with premature yellowing and plants with dying or dead shoots. A sample of ginger plants
showing disease symptoms was dug up and the rhizomes collected for isolation of pathogens. Rhizome
pieces were thoroughly washed with tap water and then dipped in 95% (v/v) ethanol and flamed quickly.
For fungal isolations, each rhizome piece was cut with a sterile scalpel and tissue with obvious
discolouration plated on potato dextrose agar + streptomycin (100 ug/mL) or corn meal agar containing
polymixin, penicillin and pirmaricin (Eckert and Tsao 1962). Bacteria were isolated by crushing pieces of
surface-sterilised rhizome with obvious disease symptoms in vials of sterile distilled water and
subsequently streaking the suspension on to sucrose-peptone agar. Bacteria and fungi from the isolation
plates were purified and stored appropriately for subsequent identification and testing.
Disease status of newly planted seed pieces. During the period from September to November 1998, forty
randomly selected seed pieces were dug upfromseveral newly-planted crops on eight farms. Seed pieces
were assessed for disease and fungal and bacterial pathogens were isolatedfroma sample of diseased
pieces and stored as above. Crops were assessed about one month after planting (time 1), and again about
one month later (time 2).
Disease status of visually healthy ginger seed pieces. Material for this experiment came from a farm that
had grown ginger for at least 15 years. The rhizome pieces were collected from a bin of washed ginger that
had a moderate to severe level of damaged knobs due to harvesting and mechanical washing. There was
evidence of discolouration on the broken ends of some of the rhizome pieces. Randomly selected rhizome
pieces were broken to obtain seed pieces because cutting may have resulted in cross contamination via the
knife blade. Each seed piece was assessed for visible signs of disease or discolouration and rated as visibly
diseased, suspect and not diseased. Care was taken to label appropriately all pieces that came from each
rhizome so that identification of their position within a rhizome could be made at the end of the experiment.
A total of 75 seed pieces (comprising 20 rhizome sections) were then planted in sterile potting mix. Seed
pieces were assessed for rotting after one month. Fungi and bacteria were isolated and stored as previously
described.
12
Disease status of seed pieces from different farms. Seed pieces were sampled from growers' storage
bins prior to planting and fungi present as surface contaminants were isolatedfroma random sample. Seed
pieces were then planted in virgin soil at Moggill, Queensland. Plants were dug up six weeks later and
assessed for disease. The fungi isolated from the seed pieces were tested for pathogenicity using protocols
detailed later.
1999 surveys of diseases in the field. In March 1999, ginger plants with at least one dead or yellowing
shoot (similar to early symptoms caused by Foz) were dug up in several fields and rhizomes were checked
for disease/discolouration.
The same fields were visited in late April 1999 and assessed again for disease. At this time, two locations
in a field were selected at random, rows of ginger were separated into 10 m lengths and a plant was selected
for assessment every 4-5 m. Twenty plants were assessed in this manner for evidence of shoot death.
Plants with obvious symptoms were dug up and rhizomes collected for isolations. Unfortunately, some of
the fields visited at early March had been harvested and therefore other similar fields had to be substituted
in the second stage of the survey.
Disease status of seed pieces in 1999. Seed pieces were collected from storage bins on several farms and
planted in the field at Moggill in an area never previously planted to ginger. Plants were harvested after 10
weeks and rhizomes assessed for disease. Isolations were carried outfromdiseased tissue in all of the
above surveys.
Results
1998 surveys of diseases in the field. The results (Table 1) show that Fusarium oxysporum f. sp. zingiberi
(Foz) was the most commonly isolated fungus from diseased rhizomes dug up from seed patches. Another
Fusarium sp. was also isolated and had characteristics similar to Fusarium solani (Burgess et al. 1994).
Pythium spp. were isolated at a much lower frequency and Sclerotium rolfsii, Geotrichum sp., Rhizoctonia
sp. and the bacterium Erwinia sp. were each isolated from one farm.
Table 1. Disease levels in seed patches in April-May 1998, and the pathogens associated with the
diseased rhizomes.
Farm
location
Noosa
Eumundi 1
A
B
A
B
A
Total no of metres of
row examined
153
153
30
50
13
No. of plants with
disease symptoms
33
133
106
108
26
Eumundi 2
B
C
A
16
19
18
48
39
50
A
B
A
B
A
16
13
5
5
5
35
32
5
20
2
Yandina 1
North Arm
Beerwah
Nambour
Site ID
13
Organisms isolated
Foz; Fusarium sp.
Foz; Fusarium sp.; Pythium
Foz; Fusarium sp.
Geotrichum; Rhizoctonia
Foz; Erwinia; Fusarium sp.
Erwinia; Fusarium sp.
Foz; Fusarium sp.; Pythium;
Sclerotium rolfsii
Foz
Foz
Foz; Pythium
Foz
Foz
Disease status of newly-planted seed. Results (Table 2) showed that the severity of seed germination
problems varied from farm to farm. On most farms, seed losses were less than 20% but losses on a few
farms were greater than 50%. In most cases, disease incidence was greater two months after planting than
it was after one month. The main symptom observed were seed pieces with buds that were blackened and
diseased. When rot was just beginning, the ginger tissue was firm and only the vascular strands were
affected. In the advanced stages of the disease, the tissue became soft and mushy and some pieces had a
strong offensive odour.
Isolations from diseased seed pieces revealed that Foz was associated with poor emergence at all
monitoring sites. The soft rot bacterium Erwinia sp. was also isolated from diseased pieces but its
distribution was less consistent. Erwinia was not detected on three of the eight farms. Sclerotium rolfsii
was isolated at a low frequency from two farms. Although Pythium was isolated from diseased rhizomes in
the May 1998 survey, it was not subsequently detected in diseased seed pieces. A bacterium (Gramnegative, fermentative) was commonly isolated from seed pieces that had been colonised by Erwinia sp. or
fungi.
Disease status of visually healthy ginger seed pieces. All 'suspect' seed pieces developed rotting, as did
many of the pieces designated as healthy (Table 3). The causal agent was Foz in all cases. Only ten pieces
out of 33 that were apparently 'healthy' did not develop Fusarium rot.
Table 2. Disease status of fungicide-treated seed pieces planted on various farms in 1998.
Location of farm
Percentage of seed pieces that did not emerge due to disease
Site!
Site 2
Site 3
Timel
Time 2
Time 1
Time 2
Time 1
0
2
nd
nd
0
12
2
0
nd
5
10
0
30
62
15
0
0
5
nd
14
7
19
18
62
45
0
nd
58
17
nd
37
45
North Arm
Yandina 1
Yandina 2
Yandina 3
Eumundi 1
Eumundi 2
Noosa
Beerwah
# nd = not done
Site 4
Time 2
Time 1
Time 2
nd
nd
nd
nd
nd
nd
nd
22
12
nd
nd
nd
nd
41
nd
nd
nd
14
23
nd
nd
nd
14
74
nd
10
nd
12
nd
nd
nd
nd
Table 3. Disease development in seed pieces that were inspected at the time of separation from the
rhizome and rated as obviously diseased, 'suspect' or not diseased.
Obviously diseased seed pieces
(22 pieces planted)
No. Diseased
No. Healthy
22
0
Suspect seed pieces
(20 pieces planted)
No. Diseased
No. Healthy
20
0
Apparently healthy seed pieces
(33 pieces planted)
No. Diseased
No. Healthy
23
10
Disease status of seed pieces from different farms. Most batches of seed had high levels of Foz (Table
4). However, two seed sources (Eumundi 1 and Nambour) were relatively uncontaminated. Geotrichum
sp. and Penicillium sp. that occurred as surface contaminants were not pathogenic to ginger (see
pathogenicity tests in the next section).
14
Table 4. Level of disease in batches of seed from different farms in 1998. The ginger was in storage
prior to planting in the field.
Farm /history of seed pieces
Noosa ("ex. Canton mother seed natch, over 2
weeks in storage, dipped in carbendazim.
Fungal contamination visible on cut surfaces).
Beerwah (Queensland, dipped in carbendazim
and stored for 1 month).
Eumundi 2 ("Canton, dinned in carbendazim
and stored for 7 weeks. The knob ends looked
diseased) This ginger was not planted out in the
field but stored in paper bags.
Eumundi 1 (Canton, dipped in carbendazim
and stored for 1 month).
Nambour (Queensland, diroed in benomvl
and stored for one month).
Beerwah (Canton diroed in carbendazim and
stored for more than 2 weeks).
Number of diseased seed
pieces/total number of seed
pieces planted
46/100
Fungi isolated from
stored ginger
44/50
Foz
54/70
Foz and Erwinia
3/34
Penicillium and Foz
0/50
-
20/57
Penicillium and Foz
Geotrichum and Foz
1999 surveys for diseases in the field. The levels of Foz in rhizomes in early March ranged from high to
none (Table 5). Infection varied from one farm to another and also within fields on single farm. Not all
plants with dead or yellowing shoots yielded Foz. However, no other pathogens were isolated. The survey
in April 1999 showed that the levels of Foz-infected ginger had increased in the 6 weeks since the March
survey (Table 6).
Table 5. Incidence of disease in rhizomes at early harvest from plants that had at least
one yellowing or dead shoot.
Farm
Site
Beenvah
A
B
C
D
No. of plants dug up (all had
symptoms)
8
5
6
4
Eumundi 1
A
B
C
D
9
2
9
5
0
50
0
0
Yandina 1
A
4
100
Noosa
A
3
0
15
% plants infected with Foz
38
60
17
25
Table 6. Incidence of Foz in seed patches on three farms in late April 1999
Farm
Site
Beerwah
A
B
C
Total no. of plants observed for
disease (healthy + diseased)
20
20
20
% plants with Foz
Eumundi 1
A
B
C
20
20
20
25
60
35
Noosa
A
B
20
20
10
15
55
70
80
Disease status of seed pieces in 1999. In general, levels of disease in ginger seed pieces were less in 1999
(Table 7) than in 1998 (Table 4). The predominant pathogens isolated were Foz and E. chrysanthemi.
Table 7. Levels of disease in a random sample of ginger seed pieces collected from storage bins on
several farms. The seed pieces were planted in Foz-free soil and harvested 10 weeks later.
Farm
Seed batch
Total number planted
No. of diseased seed pieces
1
2
100
100
6
19
3
103
15
Noosa
1
100
4
Eumundi 2
1
100
5
Beerwah
1
113
7
Yandina 2
1
103
7
Eumundi 1
Discussion
The results from surveys of potential seed patches and young plantings in 1998 and 1999 showed that the
main organism associated with poor emergence was Foz. This fungus was isolated from 12 out of 13 fields
(located on several farms) that were designated as sources of planting material in 1998. However, the
frequency of isolation of Foz varied between farms and between different fields on any one farm. Foz was
also the main organism isolated from diseased ginger seed pieces collected from 20 fields on several farms
soon after planting in October-November 1998. Foz was detected on all farms but the levels varied
between farms. Given that Foz was present in all potential "seed patches" in May 1998, the results from
young plantings suggest that there was considerable carryover of contaminated material during seed
preparation.
The fact that seed pieces can be contaminated with Foz but may appear healthy was demonstrated
experimentally. Twenty-three out of 32 seed pieces that were apparently healthy subsequently developed
Fusarium-rot. The pathogen was most likely present as a surface contaminant (mycelium or conidia) from
infected material, or alternatively, low numbers of micro conidia were present in the vascular strands.
These low levels of infection subsequently caused rotting in the ginger.
16
Although disease levels in seed pieces were generally less in 1999 than 1998, Foz was still the predominant
pathogen. A range of other fungi was also isolated from diseased and discoloured tissue, including
Pythium spp., Rhizoctonia, Sclerotium rolfsii, Geotrichum and two other Fusarium spp. The bacterium
Erwinia was present on some farms and a Gram-negative fermentative bacterium was commonly isolated
from all farms. In many instances, Foz and Erwinia occurred together. The pathogenicity and relative
importance of all of these organisms is discussed in the next two sections of this report.
Contribution of Foz in soil to poor seed emergence
Methods
Soils from different farms. Soil was collected from eight fields with different cropping histories on five
ginger farms. Half the soil was heat-treated (70°C for 3-4 days) to eliminate pathogens. Untreated or heattreated soil (1.2 L) was then added to 1.5 L plastic pots and ginger seed pieces that were not dipped in
fungicide were planted. The ginger used in the experiment was obtained from seed patches on the same
farm as the soil was collected. Twenty-five replicates were set up for each treatment. Pots were placed in a
shadehouse and the soil kept moist. After three months, plants were harvested, rhizomes were rated for
disease and pathogens were isolated and stored.
Soils from the same farm but with different cropping histories. For this experiment, soil was selected
in December 1998 from two sites A and B on the same farm at Eumundi. Observations from two field
experiments had shown that the untreated seed pieces planted at site A had much lower levels of disease
than seed pieces planted at site B. This experiment was therefore set up to determine whether this disparity
was due to differences in levels of Foz and /or other pathogens at the two sites, or to differences in seed
infestation between the two batches of seed that were used in thefieldtrials. Soil from each site was potted
and ginger seed pieces from the same batch and not treated with fungicide were planted. Ginger from the
same batch of seed was also potted in sterile peat-sand mix to determine background levels of disease.
Thirty replicate pots were used for each treatment. Plants were harvested after 10 weeks and rhizomes
assessed for disease.
Results
Soils from different farms. There was little difference in the number of diseased pieces or % rhizome rot
between untreated and heat-treated soil from any one site (Table 8). Foz was the main pathogen isolated
from diseased seed pieces. E. chrysanthemi was isolated at a low frequency from pieces grown in some
soils, but Foz was also present in most of the seed pieces colonized by the bacterium.
Soils from the same farm but with different cropping histories. Ginger seed pieces planted in the soils
and peat-sand mix developed some disease (Table 9). The moisture content of the potting-mix was higher
than that of thefieldsoils and this may have been the reason why a slightly higher number of seed pieces
from this treatment were diseased. Nevertheless, the results confirmed that pathogens in soil contributed
little to the poor emergence of seed pieces. The pathogens isolated from diseased tissue were Foz and E.
chrysanthemi.
17
Table 8. Effect of planting growers' seed pieces in heat-treated and untreated soil from the farm from which
Farm
Site
Details
Number of
disease/total
piec
Untreated
soil
7/25
Cemetery
patch
Continuous ginger for several years and has a Foz problem.
Fumigated with metham sodium in 1998.
North Arm
patch
Ginger for 3-4 years. Market ginger harvested in September
1998. Left over ginger was ploughed out and the soil
fumigated with metham sodium. Several pathogens were
isolated from ginger debris.
2/24
Beerwah
Mango patch
Ginger in 1997 had Foz problem. Fumigated with metham
sodium in 1998.
11/28
Nambour
Block 1
Ginger grown for 35 years but cropped only 1 in 4 years.
Pasture in non-ginger years. Never fumigated.
4/20
Eumundi 2
Opposite big
tree
Ginger in previous year, some Foz present. Fumigated with
metham sodium in 1998.
22/23
Yandina 1
Steel patch
Ginger in 1997, then fallow for 3 months. Fumigated with
metham sodium in 1998.
4/14
Patch with
electricity
pole
Near mango
trees
Only 20% germination in 1997. Fumigated with metham in
1998. Soil was collected mainly from a non-fumigated row.
3/15
Ginger last year. Fumigated with metham sodium in 1998.
2/25
Eumundl 1
North Arm
18
Table 9. Disease levels in seed pieces planted in three different soils (A and B were from the same farm and
the other soil was pasteurized potting mix).
Treatment
Site A
Site B
Potting mix
History
Long history of ginger. Previous crop was
early harvest. Therefore block had a
longer fallow than site B. Not fumigated.
3-4 years in ginger. 1998 crop harvested
in September. Much ginger debris
ploughed in and soil fumigated with
metham. Next crop planted soon after.
Several pathogens isolated from debris.
Pasteurised sand-peat mix.
Mean no.
of healthy
shoots
No. of
diseased seed
pieces
Mean %
rhizome rot
0.93
10
16.8
0.97
8
15.3
0.87
13
16.8
Discussion
The results of the two experiments showed conclusively that Foz in soil had little impact on seed piece rot. This
suggests that poor emergence is due largely to contamination of seed by one or more pathogens. The rapidity and
the extent to which this rot occurs depends on the level of contamination and the prevailing weather conditions. The
results also demonstrated that the levels of disease in seed pieces varied considerably from farm to farm.
Pathogenicity tests with fungi and bacteria
Methods
A series of experiments were carried out on bacterial and fungal isolates collected in 1998 and 1999.
Fusarium. For all Fusarium isolates, cultures were grown on potato dextrose agar (PDA; g/L of distilled water:
potato 200 g; dextrose 20 g and agar 18 g) for 7 days. Spore suspensions were prepared in sterile water, filtered
through gauze and ginger seed pieces (5 per isolate) were dipped in the suspension (1 x 106 spores /mL) for five
minutes. The treated seed pieces were then planted in pasteurised peat-sand mix. Seed pieces dipped in sterile
water were used as controls. After incubation in a constant environment glasshouse (23-25°C night/ 26-28°C day)
for two months, seed pieces were assessed for shoot emergence and disease. Several isolates were tested twice.
Pathogens were re-isolated on PDA + streptomycin.
Pythium. Pythium sp. isolates were cultured in sterile cornmeal sand mix (3 g cornmeal and 100 g washed river
sand mixed with 15 mL water in glass jars and autoclaved for 20 min on two consecutive days). Each jar was then
inoculated with an isolate and incubated at 25°C for two weeks. The cornmeal-sand inoculum was incorporated in
sterile potting mix at two inoculum densities (1 g/L or lOg/L of mix). Ginger seed pieces were planted in pots placed
in a constant environment glasshouse and pieces were assessed for emergence and rotting after two months.
Treatments consisting of potting mix amended with sterile cornmeal sand alone served as controls.
Geotrichum and S. rolfsil Seed pieces (half with buds intact and half with buds that were damaged by rubbing
with a coarse carrot grater) were dipped for 10 min in suspensions (10s spores/ mL) of Geotrichum sp. growing on
PDA for 5 days. Pieces were air-dried for 30 min and then sealed in plastic bags and incubated in the laboratory at
ambient temperature. After one month, seed pieces were assessed for rotting. Seed pieces dipped in sterile water
were used as controls.
Bacteria. For testing the Gram-negative, fermentative bacterial isolate, seed pieces were dipped in a suspension
(109 cfu/mL) of bacteria grown on sucrose-peptone agar (Fahy and Hayward 1983) for 48 hours. Seed pieces were
then incubated in two ways. Half the rephcates were planted in pasteurized peat-sand and placed in a glasshouse at
23-28°C. The other half were placed in plastic bags and incubated in the laboratory at ambient temperature. Seed
pieces were checked for rotting after 2, 4 and 6 weeks. Seed pieces dipped in sterile water were used as controls.
19
Since temperature, moisture and the level of injury can affect the pathogenicity of Erwinia, experiments were set up
to test all of these parameters. In all four experiments, seed pieces were dipped for 10 min in a bacterial suspension
(109/ mL) prepared from cultures grown on sucrose peptone agar. In the first experiment, treated and untreated seed
pieces were sealed in plastic bags and incubated at ambient temperature (18-23°C) or at 30°C. Ginger was assessed
for rotting after one month. In the second experiment, ginger was planted in pasteurized potting mix and pots placed
at 27-28°C or 33-34°C and assessed for rotting after 2 weeks. In the third experiment, inoculated and uninoculated
seed pieces were planted in pasteurized potting mix and incubated at ambient temperature or at 30°C. Half the
replicates of each treatment were allowed to drain after watering (dry) and the soil in the other half was maintained
at saturation (wet) by placing pots in saucers of water. In the last experiment, seed pieces with one or two cut
surfaces (to simulate different levels of injury) were used. Seed pieces were incubated for 2 weeks in plastic bags at
28-30°C and then assessed for rotting.
Interaction of Foz, E. chrysanthemi and Enterobacter sp. Because these three organisms frequently occurred
together in rotting ginger, an experiment was set up to test whether disease was exacerbated because of an
interaction between them, and whether soil moisture increased disease severity. Ginger seed pieces were dipped in
the organisms alone or in various combinations for 5 mins. Thefinalconcentration of Foz in the suspension was 106
spores /mL and of each bacterium was 108 cfu/mL. Treated seed pieces were planted in pasteurized peat-sand mix
in 1 L pots. Seed dipped in water were set up as controls. The pots in half the replicates from each treatment were
allowed to drain freely after watering, whereas the other set of pots was placed in saucers so that free drainage did
not occur. All pots received the same amount of water daily. Plants were grown in a shade house for 10 weeks and
then harvested. Rhizomes were rated for % rot and the number of healthy and diseased shoots noted.
Susceptibility of ginger cv. Canton and cv. Queensland to Foz, Ginger seed pieces of both cultivars were
carefully prepared from hand-harvested ginger and then dipped in suspensions (103 and 106 spores/mL) of Foz. The
seeds were then planted in potting mix, the pots placed in a shade house and watered normally. Seed pieces dipped
in water were used as controls. Fifty replicate seed pieces were set up for each treatment. Half the replicates were
harvested after two months and the other half after four months. The number of healthy shoots and the % disease in
rhizomes was noted.
Results
Fusarium. A total of 37 isolates were tested and 28 of them were pathogenic to ginger. The symptoms were a
brown discolouration of the rhizome accompanied by some shriveling. A few seed pieces had advanced symptoms
where only a shell remained in which the fibrous tissue persisted. Most seed pieces did not produce a shoot, but if a
shoot was produced it soon turned yellow, wilted and eventually died.
Erwinia. The isolate of Erwinia produced a soft, mushy rot with a characteristic strong offensive odour. The
disease sometimes developed within a week or sometimes took longer, depending on temperature and moisture
conditions. The control seed pieces in all four experiments remained healthy. The results (Table 10) demonstrate
that temperature, moisture and the number of cut surfaces all affect rotting of ginger seed pieces infected with
Erwinia.
Pythium. None of the five isolates tested were pathogenic to ginger.
Sclerotium rolfsii. The single isolate tested rotted ginger seed pieces within one month. The affected tissue had a
faint pinkish-yellow tinge and the coarse mycelium of the fungus was clearly visible. Abundant sclerotia were
produced within 4-6 weeks.
Geotrichum. None of the isolates of Geotrichum rotted ginger seed pieces, even when the tissue was damaged.
Enterobacter sp. This bacterium did not rot ginger seed pieces.
20
Table 10. Experiments with Erwinia (Erw) showing effect of temperature,
soil moisture and level of injury on disease severity in seed pieces.
Experiment no./
medium
1. Plastic bags
2. Potting mix
Percentage of seed pieces that were rotten
+ Erw(18-23°C)
70
+ Erw (29-30°C)
90
+ Erw (27-28°C)
35
+ Erw(33-34°C)
85
(18-23°C)
+Erw wet
+ Erw dry
40
0
(29-32°C)
+Erw wet
+ Erw dry
40
0
+ Erw (1 cut surface)
50
+ Erw (2 cut surfaces)
80
3. Potting mix
4. Plastic bags at
25-30°C
NB: In all four experiments, the control seed pieces were healthy.
Interaction of Foz, E chrysanthemi and Enterobacter sp. Ginger seed pieces used in this experiment had low
background levels of Foz contamination. Unfortunately this is inevitable, as the industry has no sources of
completely disease-free planting material. Therefore, some disease due to Foz developed in the treatments that were
not inoculated with this pathogen (Table 11). Notwithstanding this, the results clearly demonstrated that the type of
organisms and the moisture content of the soil had a significant effect on disease severity. There was also a
significant interaction between organism type and soil moisture. Rhizome rot caused by E. chrysanthemi was worse
with high soil moisture. When Foz was also present, nearly 100% of all rhizomes were rotted. The impact of
Enterobacter sp. appears to be minimal, but background levels of Foz in the seed pieces hampered interpretation of
the results. Nevertheless, Foz was clearly the most damaging pathogen (Table 11) and moisture had Utile effect on
disease severity when this pathogen was present on its own.
Table 11. Interaction table showing the effect of two different soil moisture contents on the development of
rhizome rot in ginger when seed pieces were treated with Foz, E. chrysanthemi or Enterobacter sp. alone or in
various combinations
Treatment
% rotting in rhizomes
Control (water dip)
Enterobacter sp.
Foz
E. chrysanthemi
Foz + Enterobacter sp.
Foz + E. chrysanthemi
Enterobacter sp. + E. chrysanthemi
LSD (P=0.05)
High moisture
0.29 (8.2)
0.12(1.4)
0.93 (64.3)
0.49 (22.2)
0.92 (64.3)
1.38 (96.4)
0.65 (36.6)
Low moisture
0.13 (1.7)*
0.01(0.0)
0.84 (55.4)
0.0 (0.0)
0.64 (35.7)
0.54 (26.4)
0.17 (2.9)
0.37
* Data were analysed after angular (arcsin) transformation. Values in parentheses are equivalent means.
21
Discussion
The majority of Fusarium isolates presumptively identified as Foz were pathogenic to ginger and the symptoms they
produced in seed pieces were typical of those described earlier. Some of the non-pathogenic isolates of Fusarium
were F. solani and saprophytic strains of F. oxysporum (Burgess et al. 1994). They were most likely secondary
colonisers of diseased tissue.
All the Pythium isolates were not pathogenic to ginger and it seems highly unlikely that this fungus is involved in
poor emergence. The isolates tested were obtainedfromrhizomes in seed patches but this fungus was never isolated
from newly planted seed pieces or seed pieces in storage. Several species of this fungus have been reported as
pathogens of ginger in the field under wet conditions in India (Dake and Edison 1989) and Hawaii (Trujillo 1964)
and in stored ginger in South Africa (Grech and Swarts 1990) and Australia (Persley 1994).
Although Sclerotium rolfsii was isolated from ginger and caused rotting of seed pieces in the pathogenicity test, all
observations suggest that it is a minor contributor to rotting of seed pieces. Its mycelium is distinctive and it was
never observed in rotting seed pieces. Geotrichum sp. was isolatedfromrotting ginger, but its pathogenicity was not
established in two tests, even when ginger seed pieces were injured by rubbing them against a coarse vegetable
grater. There is one record of Geotrichum causing rotting of stored ginger collectedfrommarkets in Orissa, India
(Mishra and Rath 1989). These authors demonstrated in subsequent pathogenicity tests that ginger rotted after two
weeks when the temperature was maintained 25°C and the relative humidity was 100%.
E. chrysanthemi is probably involved in the seed piece disease complex but it was not isolated from all farms.
Pathogenicity experiments showed that E. chrysanthemi on its own can cause disease in ginger seed pieces and that
the severity of disease was exacerbated by increasing soil moisture and temperature. The amount of exposed
rhizome tissue following injury (simulated experimentally by using seed pieces with one or two cut surfaces) also
affects disease severity. In diseased seed pieces collected from surveys and from the biocide trails, E chrysanthemi
was nearly always associated with Foz. It is therefore likely that ginger damaged mechanically during harvesting
and seed preparation and /or infected with Foz or damaged by root-knot nematodes (Meloidogyne spp.) is vulnerable
to colonisation by E. chrysanthemi (Pegg et al. 1994). Evidence from an experiment that studied the effect of Foz
and E. chrysanthemi when applied on their own or in combination, confirmed that seed piece rot was more severe
when seed pieces were treated with both organisms than when E. chrysanthemi was inoculated alone.
Although Enterobacter sp. was commonly isolated from ginger infected with Foz and/or E chrysanthemi, its
pathogenicity was not established. To date, Enterobacter has never been shown to be a significant plant pathogen.
In fact, several Enterobacter sp. have shown promise as biological control agents against fungal plant pathogens
such as Pythium spp. (Howell et al. 1988; Nelson 1988). It is unlikely that this bacterium is involved in poor
emergence of ginger.
At the start of this project, some ginger growers claimed that the problems of poor emergence became more
significant following a changefromcv. Queensland to cv. Canton. Since most growers intended to increase their
plantings of cv. Canton, experiments were carried out to test this hypothesis. Both ginger cultivars (Queensland and
Canton) proved to be equally susceptible to Foz, as disease levels were similar in the two cultivars four months after
inoculation with the fungus. Canton ginger has larger knobs than Queensland ginger, and therefore appears to be
more prone to damage during harvesting and washing operations. Damaged surfaces (in disease-free ginger) are
vulnerable to infection from fungal spores present on other infected seed pieces during the seed preparation-storage
stage. Therefore, Canton pieces are more likely to become infected with Foz prior to planting and infection prevents
budsfromdeveloping and reduces the number of shoots produced. This may be why there appears to be a greater
problem with seed piece emergence in Canton rather than Queensland ginger, and why more areas of bare ground
are seen in young Canton plantings. Queensland ginger often emerges well but the shoots wither and die later in the
season (i.e. from April onwards).
22
Identification of bacteria and fungi
Methods
All Fusarium isolates were identified to species level using protocols described in Burgess et al (1994). The fungi
presumptively identified as Pythium (based on the morphology of colonies) were cultured on corn meal agar
(commercial preparation) to induce formation of reproductive structures. The identity of Sclerotium rolfsii was
confirmed using morphological characteristics when cultured on PDA. Geotrichum sp. was identified on colony
appearance and conidial morphology on PDA. The species of the Erwinia was confirmed using the potato shce test,
appearance on PDA and pigment production on calcium carbonate-yeast extract-glucose agar (CYDA) (Fahy and
Persley 1983). The GN Microplate™ identification system (BIOLOG, Hayward CA) was also used. The
characteristics of the Erwinia isolates from ginger were compared with known species of Erwinia, UQM 225 (K
carotovora pv. atroseptica) and UQM 2193 (E. chrysanthemi) obtained from the Bacterial Culture Collection
(Department of Microbiology, The University of Queensland). The Gram-negative, fermentative bacterium was
identified to genus using the BIOLOG system as above.
Results
All of the Fusarium isolates that were pathogenic were confirmed as Foz. Four of the non-pathogenic isolates had
characteristics similar to F. solani whilefiveresembled Fusarium oxysporum.
The Pythium isolates were confirmed as belonging to this genus after observing oospores on CMA. Since none of
the isolates were pathogenic, no species determination was carried out.
The presence of small, dark-brown, globose sclerotia and clamp connections in hyphae confirmed the identification
of Sclerotium rolfsii.
Isolates designated as Geotrichum sp. were verified by the absence of conidiophores and presence of characteristic
arthrospores (conidia, one-celled short, cylindrical with truncate ends) (Barnett and Hunter 1998).
The Gram-negative, fermentative bacterium was identified as Enterobacter sp. with a similarity index of 0.873 in
the BIOLOG system.
The characteristics of the Erwinia isolates are detailed in Table 12, along with the results of the BIOLOG test. All
isolates were identified as E. chrysanthemi. This identification was further confirmed by the potato soft rot test and
the appearance of the isolates on PDA and CYDA (Fahy and Hayward 1983). Fatty acid analysis (FAME) carried
out by Dr E Cother (Australian Collection of Plant Pathogenic Bacteria (Herb. DAR)) also confirmed the species as
Erwinia chrysanthemi.
23
Table 12. Characteristics of the Erwinia isolates obtained from ginger.
Isolate No.
Gin 15*
(DAR 73905)
Gin 23*
(DAR 73906)
Gin 24
Gin 38
Gin 39*
(DAR 73907)
Gin 40
Gin 51
Gin 52
Gin 53*
(DAR 73908)
UQM 225
E carotovora
pv.
atroseptica
UQM 2193
E.
chrysanthemi
Appearance on PDA
Appearance on
CYDA
Dark blue
pigment after 4
days
Potato
slice test#
++
BIOLOG
Similarity IndexH
0.839
++
0.737
Cream colonies with undulate
margins, umbonate, darker in
centre "fried egg" appearance
(in some colonies)
As above
++
++
++
0.809
As above
As above
As above
As above
As above
As above
++
++
++
++
0.835
0.813
0.832
0.84
As above
As above
As above
As above
As above
As above
As above
As above
+
0.476
margins only
No pigment
produced
+
0.683
margins only
No pigment
produced
As above
# ++ = potato slices completely rotted within 18 hrs; + = potato slices completely rotted after 36 hrs
+
The unknown strain is compared to all strains of the suggested species within the BIOLOG data base. The
Similarity Index is the calculated value supplied by BIOLOG software that indicates how closely related the
unknown isolate is to its suggested identity.
* ID confirmed by fatty acid analysis. Cultures have been lodged with 'The Australian Collection of Plant
Pathogenic Bacteria (Herb. DAR)'.
Experiments with biocides
Pot andfieldexperiments were carried out in 1998 and 1999 to test the effect of a range of biocides on emergence
and disease development in ginger seed pieces. The biocides were mainly used as tools to ascertain which pathogens
may have been responsible for poor emergence and disease of seed pieces. Because both fungi and bacteria may
have been involved, biocides were selected that had activity against either fungi, bacteria or both groups of
pathogens.
Methods
The biocides selected and the rates used in all experiments were as follows: Benlate®(Dupont) applied as 250ng/mL
benomyl, Bavistin FL®(BASF) applied as 1000 ug/mL carbendazim, Dry Bordeaux ™ (Chemspray) applied as
1500 ug IvaL copper hydroxide, Cuprox ™ applied as 1500 jig/rnL copper oxychloride, Amistar (Cropcare)™
applied as 125 ug/mL azoxystrobin, Ridomil, 250EC™ (Ciba-Giegy) applied as 375 jig/mL metalaxyl and calcium
hypochlorite applied at 100 ug/mL.
Pot experiments in 1998. Ginger rhizomes were harvested from two different farms (Yandina 1 and Noosa) and
washed before cutting and dipping in the various biocides for 5 minutes. Treated seed pieces were air-dried and
stored for 3 days before planting in IL pots containing pasteurised potting mix. Twelve replicate pots were set up for
each treatment. Seed pieces dipped in water alone were set up as controls. Pots were placed in a glasshouse at 2324
27°C and watered regularly. Plants were harvested after 3 months and rhizomes rated for rotting. A sample of
diseased rhizomes was collected for isolation of pathogens using the protocols described previously.
Field experiments in 1998. Several experiments carried out on growers' properties also tested the effect of the
biocides listed above. Seed pieces collected from the growers' cutting tables were dipped in the various biocides
and stored in brown paper bags until planting about one week later. All trials were planted by hand and were
located on farms where the seed pieces were obtained. A control treatment of untreated seed dipped for 5 min in
water was planted in every trial. Ginger plants were assessed for disease approximately ten weeks after planting by
counting the number of healthy and diseased shoots in each plot. Following this, all plants in 0.5 m of row at the
beginning and end of each plot were dug up and the rhizomes rated for disease. In some experiments the remaining
ginger was used to assess the number of healthy and diseased clumps approximately six months after planting. A
diseased clump was one that had at least one yellowing or dead shoot. Pathogens were isolated from a sample of
diseased rhizomes after ten weeks and seven months.
Experiments in 1999. In 1999, the quaternary ammonium compound didecyldimethyl ammonium chloride
(SporekiU™, Lefroy Valley) was tested for efficacy against Foz and E. chrysanthemi. For the bacterium, seed pieces
prepared from hand-harvested and hand-washed ginger were dipped in a suspension (109bacteria/mL) of E.
chrysanthemi and then air-dried. The pieces were then dipped in either SporekiU (120 ppm active ingredient),
copper oxychloride (1500 \xg/mL) or a mixture of the two for five minutes. Bacteria were extracted from a sample
of seed pieces one hour after dipping in the biocides. The remaining seed pieces were then planted in potting
medium and it was maintained at saturation by placing pots in saucers of water. Seed pieces were harvested after
two months and rated for rotting.
For the experiment with Foz, seed pieces were dipped in a suspension of spores (106 /mL of water) and then airdried. They were subsequently dipped in either the quarternary ammonium compound or copper oxychloride at the
concentrations used above, or carbendazim at 1000 |ag/mL. Seed pieces were planted in a field at Moggill where
ginger had never been planted previously. Plants were harvested after 3 months and rhizomes assessed for rotting.
In a second experiment, seed pieces from a grower's cutting table were collected prior to fungicide dipping but were
not inoculated with Foz (in contrast to the previous experiment). The seed pieces were dipped in the above
chemicals and subsequently planted at Moggill. After two months, they were assessed for disease as before.
Results
Pot experiments in 1998. The results of the two pot experiments (Table 13) showed that seed pieces dipped in
carbendazim, benomyl, or copper hydroxide developed the least amount of rotting. Metalaxyl and azoxystrobin
gave no disease control.
Table 13. Effect of various biocides on emergence of seed pieces from two farms (Yandina i a n d Noosa).
Treatment
Control
Copper hydroxide
Azoxystrobin
Metalaxyl
Hot water
Carbendazim
Carbendazim
(20 min at 48°C)
Benomyl
Benomyl
(20minat48°C)
LSD(P=0.05)
Mean number of healthy
shoots/seed piece
Yandina 1
Noosa
0.25
0.75
1.25
0.33
0.0
0.58
0.25
0.83
0.33
0.83
1.3
0.58
1.5
0.92
No of healthy seed
pieces out of 12
Yandina 1
Noosa
4
4
8
7
0
4
5
5
4
2
10
10
10
7
Mean % healthy
rhizomes
Yandina 1
Noosa
43.3
48.3
65.8
88.7*
13.3
42.5
41.7
46.2
25.8
58.7
97.1
99.2
98.3
89.2
0.83
0.58
0.92
1.17
8
6
10
8
86.7
74.2
94.6
97.5
nd
nd
nd
nd
30.8
27.9
nd = analysis not done
* Bold numbers are significantly different from the control.
25
Field experiments in 1998. The results of these experiments are detailed in Tables 14-19. They confirmed that
benomyl and carbendazim had the greatest effect in reducing disease in seed pieces. Copper hydroxide reduced
disease in one experiment (Table 16) and copper oxychloride reduced disease in three experiments (Tables 14, 17
and 18).
Table 14. Effect of seed dip treatments on disease in seed pieces from North Arm site 1. Forty seed pieces
were planted in each of five replicates.
Treatment
Mean number of
shoots per seed piece
Mean %
healthy seed
pieces
Untreated (water dip) (control)
Untreated (water dip) but planted in sawdust
Benomyl
Carbendazim
Copper oxychloride
Metalaxyl
0.86
0.78
1.12
1.11
1.11
0.71
81.1
67.5
100
100
95.0
53.9
No. of clumps of
healthy ginger/total
number of clumps
(5 March 1999)#
22/35
18/32
36/40
41/45
not done
not done
LSD(P=0.05)
0.14
21.24
-
# Clumps were rated for the presence or absence of at least one yellowing or dead shoot.
Table 15. Effect of dip treatments on disease in seed pieces from Yandina 1. Twenty-one seed pieces were
planted in each of six replicates.
Treatment
Untreated (water dip)
Carbendazim
Carbendazim (but maintained at 48°C)
Metalaxyl
Carbendazim + metalaxyl
LSD(P=0.05)
Mean number of
shoots per seed
piece (9 weeks after
planting)
0.14
0.54
0.75
0.09
0.63
Mean %
healthy seed
pieces (9 weeks
after planting)
21.5
83.1
77.7
28.6
80.6
0.27
18.91
No. of clumps of
number of clumps
(5 March 1999)*
1/2
17/20
15/21
0/5
12/20
Bold numbers are significantly different from the control.
Table 16. Effect of dip treatments on disease in seed pieces from Eumundi 1 site 2. Thirty-four seed pieces
were planted in each of five replicates.
Treatment
Copper hydroxide
Calcium hypochlorite
Carbendazim
Copper + carbendazim
Calcium hypochlorite + carbendazim
Mean number of
shoots per seed
piece
Mean %
healthy seed
pieces
0.65
0.84
0.86
1.14
1.03
1.08
63.7
85.1
68.3
78.7
84.4
72.9
No. of clumps of
healthy
ginger/total
number of clumps
(5 March 1999)*
19/25
33/40
not done
not done
39/41
not done
not significant
-
LSD(P=0.05)
0.19
26
Table 17. Effect of dip treatments on disease in seed pieces from Beerwah. Fifty
seed pieces were planted in each of eight replicates.
Mean number of
shoots per seed
piece
0.10
0.32
0.01
0.39
0.29
0.01
0.25
Treatment
Copper oxychloride
Metalaxyl
Carbendazim
Metalaxyl and carbendazim
Azoxystrobin
Carbendazim (20 min at 48°C)
Mean %
healthy seed
pieces
4.9
44.9
9.5
30.6
27.2
4.2
13.1
LSD(P=0.05)
15.6
0.09
Table 18. Effect of dip treatments on disease in seed pieces from Eumundi 1 site 3. Forty seed pieces were
planted in each of eight replicates.
TREATMENT
Carbendazim
Copper oxychloride
Copper oxychloride + carbendazim
Calcium hypochlorite
Calcium hypochlorite + carbendazim
Azoxystrobin
Azoxystrobin and carbendazim
Mean number of
shoots per seed
piece 10 weeks
after planting
0.39
1.79
1.04
1.64
0.48
1.86
0.67
1.71
12.5
78.1
51.9
84.4
12.5
96.9
43.8
90.6
No. of clumps of
number of clumps
(5 March 1999)#
6/8
38/40
25/33
32/37
9/15
33/43
13/22
32/44
22.73
nd
Mean % healthy seed
pieces 10 weeks after
planting
LSD(P=0.05)
0.20
Clumps were rated as described previously.
Table 19. Effect of dip treatments on disease in seed pieces from Eumundi 1 site 3.
Fifty seed pieces were planted in each of four replicates.
Treatment
Copper oxychloride
Metalaxyl
Carbendazim
Metalaxyl and carbendazim
Azoxystrobin
Carbendazim maintained at 48°C for 20 min
Mean number of
shoots per seed
piece
0.05
0.07
0.02
0.33
0.71
0.04
0.48
Mean%
healthy seed
pieces
0
7.5
0
38.3
47.5
2.5
32.5
0.16
26.16
LSD(P=0.05)
27
Experiments in 1999. In the experiment withi?. chrysanthemi, none of the chemicals significantly reduced the
levels of bacteria on the seed surface (Figure 1). The chemicals also did not reduce the number of rotted seed pieces
when compared to the control treatment (Figure 2). Copper was slightly more effective against disease than
Sporekill. However, only low levels of disease developed, even in the controls.
Control
Spk
Cu
Cu + Spk
Treatment
Figure 1. Number of bacteria on the surface of seed pieces after dipping them in a suspension of Erwinia
chrysanthemi, then drying them and dipping them in either water (control) copper oxychloride (Cu) or
didecyldimethyl ammonium chloride (Spk) or a mixture of the Spk + Cu for 5 minutes.
50 |
45 •
Control
Spk
Cu
Cu + Spk
Treatments
Figure 2. Percentage of seed pieces rotted by Erwinia chrysanthemi after they were dipped in a suspension of
bacteria, then dipped in either water (control) or in various chemicals and then planted in saturated potting
mix.
In the experiment with E. chrysanthemi, nearly 100% of the seed pieces rotted following treatment with the
disinfectant didecydimethy ammonium chloride (Table 20). Only the treatments containing carbendazim had at
least 30% healthy plants.
28
Table 20. The effect of didecydimethy ammonium chloride, copper oxychloride and carbendazim on seed
piece rot. Seed pieces were dipped in a suspension of Foz spores, air-dried and then dipped in the chemicals
for five minutes before being planted in the field at Moggill.
Treatment
Control (water)
Didecyldimethyl ammonium chloride
Copper oxychloride
Carbendazim
Didecyldimethyl ammonium chloride + carbendazim
Copper oxychloride + carbendazim
Didecyldimethyl ammonium chloride + carbendazim + copper oxychloride
LSD (P=0.05)
Mean % healthy
seed pieces
0
2
2
30
34
40
46
15,6
Discussion
The biocides were used as tools to help establish the causal agent/s of poor emergence. The experiments confirmed
the involvement of Foz in the problem. The two fungicides currently recommended for Foz on ginger, carbendazim
and benomyl, gave the most consistent reduction in diseased seed pieces in two pot and several field experiments in
1998 (Tables 13-19). The efficacy of these two compounds varied amongst experiments, confirming that there was
great variation in the level of disease in different batches of seed pieces. These chemicals are effective in reducing
surface contamination of ginger by Foz, but are probably less effective once the pathogen has penetrated into the
tissue. Copper based fungicides which have a general biocidal effect performed better in some experiments than
others, but gave no better control of disease than carbendazim or benomyl (see Tables 13,14, 17,18 and 19).
Since metalaxyl had no effect on disease reduction in seed pieces it is unlikely that Pythium is involved. This
confirms the results obtained in the pathogenicity tests.
Pathogenicity tests (see previous section) showed that K chrysanthemi on its own was able to rot ginger when soil
was saturated. Due to excessive rain in the spring of 1998, the soil in many of thefieldexperiment sites remained
wet for long periods of time. Since the bacterium was present on some of the farms, the reduction in rhizome rot
achieved by treatment with copper in some of the field experiments may have been because this biocide had some
effect in controlling the bacteria. In most of the trials, E. chrysanthemi was often associated with Foz-infected seed
pieces. Therefore, it is highly likely that the bacteria also readily colonised damaged or debilitated ginger. Rotting
of the ginger would then have progressed rapidly as long as conditions were conducive.
Effect of temperature and moisture on ginger in storage
Methods
Effect of storage after washing. Ginger (harvested from a field that had not been planted to ginger in the previous
season) was washed and stored in paper bags at ambient temperature. The rhizomes had a moderate level of damage
due to mechanical harvesting and washing. Rhizomes were cut into seed pieces one, four and six days later. Half of
each batch was dipped in carbendazim. Once all the seed pieces had been prepared, they were planted on a farm in
Noosa. After 10 weeks, the number of healthy shoots in each plot was noted. Following this, all plants in 0.5 m of
row at the beginning and end of each plot were dug up and the rhizomes were rated for disease. Isolations were
done from a sample of diseased tissue. Three months after the first assessment, all remaining plants in some
treatments were assessed for disease.
29
Effect of moisture. Ginger seed pieces treated with carbendazim or left untreated were stored in plastic bags after
spraying once with water (termed the moist treatment). An equal number of seed pieces were not sprayed with water
and stored in paper bags (dry treatment). Both batches of ginger were then placed at ambient temperature. This
batch of ginger was moderately to severely damaged due to mechanical harvesting and washing. After 2 weeks
storage, the ginger was planted in the field at Moggill in soil that had never grown ginger. Plants were harvested
after 10 weeks and rhizomes rated for disease.
Effect of temperature and moisture. Untreated and fungicide treated (by the grower) seed pieces were stored at
22°C or 18.5°C in paper bags (dry) or in plastic bags (moist as described above) for two weeks and then planted at
Moggill. Ginger was harvested 12 weeks later and disease was assessed in the usual way.
Results
Storage after washing. Fungicide-treated seed pieces developed significantly less disease than the untreated seed
pieces (Table 21) at all three storage times. Overall, the amount of disease in fungicide-treated seed pieces
increased as the storage time prior to dipping increased. The main pathogen isolated was Foz. E chrysanthemi,
when present, was nearly always associated with Foz.
Table 21. The effect of different post-washing storage times on the development of disease in ginger seed
pieces. Each replicate consisted of 25 seed pieces.
Treatment
Mean
number of
shoots per
seed piece
Mean %
healthy
seed pieces
Seed cut and dipped in carbendazim 24 hr after washing
Seed cut and dipped in carbendazim 3 days after washing
Seed cut and dipped in carbendazim 6 days after washing
0.92
0.62
0.34
62.5
52.8
28.8
No. of clumps
of healthy
ginger/total
number of
clumps
15/29
10/15
5/11
Seed cut and dipped in water 24 hr after washing
Seed cut and dipped in water 3 days after washing
Seed cut and dipped in water 6 days after washing
0.11
0.18
0.04
12.9
4.0
3.3
not done
not done
not done
LSD (P=0.05)
0.24
22.90
not done
#
Clumps were rated for
foi the presence or absence of at least one yellowing or dead shoot. The presence of Foz was
confirmed by isolation.
Effect of moisture. All of the seed pieces in this experiment were diseased (data not shown) regardless of the
treatment. This result shows that this batch of seed pieces was heavily contaminated with Foz at the time of
preparation.
Effect of temperature and moisture. Seed pieces dipped in fungicides had significantly less disease than those
dipped in water only (Table 22). Moisture and storage temperature did not have a significant effect on disease
levels. Foz was the main pathogen isolated but K chrysanthemi was also present in some affected seed pieces.
30
Table 22. The effect of temperature and moisture on disease development in stored seed pieces.
Treatment
Mean number of shoots
per seed piece
1.72
1.36
1.53
1.50
Mean % healthy seed
pieces
88.3
87.5
79.4
76.8
Moist at 18.5°C without fungicide
Dry at 18.5°C without fungicide
Dry at 22°C without fungicide
Moist at 22°C without fungicide
0.95
0.95
0.78
1.50
56.3
62.5
59.4
56.3
LSD (P=0.05)
0.36
1734
Moist at 18.5°C +fungicide
Dry at 18.5°C + fungicide
Moist, at 22°C + fungicide
Dry at 22°C + fungicide
Discussion
The results once again demonstrated the involvement of Foz in seed piece rot. Temperature and moisture had little
impact on disease but the fungicide (carbendazim) significantly reduced disease. During seed piece preparation,
ginger rliizomes are dug up, washed and then cut and dipped in a fungicide. Because of the high levels of disease in
some seed patches, any delay (even 1-2 days) in the sequence of steps after digging rhizomes can obviously lead to
high levels of cross contamination of previously healthy ginger with spores of Foz. Once the spores germinate and
infect the rhizome tissue, it is unrealistic to expect a protectant fungicide to give good control of disease.
Possible improved control of Foz
Effect of acibenzolar-S-methyl
Acibenzolar-S-methyl (Bion ™, Novartis) is one of a new generation of compounds that activates a plant's defense
mechanisms, resulting in a phenomenon known as Systemic Activated Resistance. This compound is not fungicidal
and therefore has no direct toxicity to fungi. However, low concentrations of the compound may provide longlasting protection against fungal pathogens by increasing resistance in the plant to attack. Recent work has
demonstrated that the chemical showed promise in glasshouse experiments against F. oxysporum f. sp. cubense on
banana (K. Pegg pers. com.). The currently registered fungicides for Foz only protect the seed piece for a few
weeks after it is planted. In contrast, a chemical such as Bion has the potential to reduce disease that develops later
in the season, thus reducing levels of Foz in the rhizomes used for seed in the next planting season. The following
experiments were designed to determine whether this control strategy had any chance of success.
Methods
In the first experiment, ginger seed pieces prepared from apparently healthy rhizomes were dipped for five minutes
in carbendazim (1000 ng/mL), Bion (0.025 g of trade product/L) or water or a mixture of carbendazim and Bion.
Seed pieces were air-dried and stored in paper bags forfivedays and then planted in pasteurised potting mix. A
spore suspension of Foz was added to the pots so that each received approximately 2 xlO5 spores /g soil. Pots were
then placed in a shade house and watered normally. After 12 weeks, plants were harvested and the % of each
rhizome that was healthy was assessed. The number of healthy shoots was also noted.
In a second experiment, seed pieces were dipped in carbendazim as the standard seed treatment. Three days later,
half the seed pieces were dipped in Bion (as above) and then all the seed pieces were planted in pasteurised peatsand mix. Pots were placed in a shade house and the ginger was grown for 10 weeks. Plants were then sprayed at
regular intervals with one of two concentrations of Bion (0.025 g or 0.050 g/L) two or three times over a period of
20 days. Details of treatments are given in Table 24. A wetting agent, Agral™ (600 g/L nonylphenol ethylene
31
oxide)(Crop Care) was used at a rate of 0.13 mL/L with all Bion sprays. Ten days after the last spray was applied,
Foz spores (106) per pot were added into a furrow made in the soil, and the soil was watered thoroughly to disperse
the spores. Eight weeks after plants were inoculated, the number of healthy shoots was noted. Plants were then
harvested, the old seed piece was removed and the new rhizomes were assessed as above. Pathogens were isolated
from a sample of diseased tissue.
In a third experiment, ginger seed pieces dipped in carbendazim were planted in pasteurised potting mix in 1L pots
and allowed to grow in a shadehouse for 12 weeks. Plants were fertilized regularly. Three Bion sprays (0.05 g/L)
were then applied 10 days apart. Plants without a Bion treatment were also included in the experiment. Soon after
the third Bion treatment, plants were transplanted into 2.5 L pots and grown for a further 8 weeks. At this stage any
plants showing obvious Foz symptoms (from seed piece contamination) were discarded. The previously sprayed
plants were divided into 2 lots of 24, and sprayed twice (10 days apart), with either 0.05 g/L or 0.025 g/L of Bion.
Control plants (24) were sprayed with water. Two days after the final spray, each pot was inoculated with Foz
spores (2.4 xlO4 /g). Plants were harvested nine weeks later and the new rhizomes were assessed as for experiment
1.
Thisfieldexperiment was carried out at Moggill and consisted of seven treatments with eight replicates arranged in
a randomised block. Each replicate contained nine seed pieces planted in 1.2 m of row. One treatment of untreated
seed pieces and six treatments of carbendazim treated seed pieces with or without Bion treatments were included.
The beds of ginger were fertilized with a regime similar to that used by growers and watered with a combination of
overhead sprinklers and trickle irrigation. All plots were inoculated with Foz sporesfivedays after the third Bion
application. Furrows were dug on either side of a row of ginger and a calibrated spore suspension sprayed into these
furrows to give a concentration of lx 109 /m2 of bed. Immediately after applying spores, they were gently watered
into the soil with a hand held hose. The furrows were thenfilledwith soil and the beds watered for 1 hr with
overhead irrigation. Plants were harvested four months after adding Foz. The number of healthy shoots was noted
and the new rhizomes were assessed as for the pot experiments. Where necessary, pathogens were isolated from a
sample of diseased tissue.
Results
Pot experiments. A single Bion dip did not reduce the level of disease in rhizomes in the first experiment (Table
23). However, a combination of carbendazim and Bion increased the number of healthy shoots. In the second
experiment where Bion was used as a spray, there was some reduction in rhizome rot (Table 24). In the third
experiment, all rhizomes from plants that hadfiveBion sprays over a period of 14 weeks were completely free from
disease (Table 25).
Field experiment In thefield,there was some reduction in disease following Bion sprays but this reduction was
not significant (Table 26).
Table 23. The effect of Bion applied as a seed dip on rhizome rot caused by Foz.
Treatment
Control (water)
Carbendazim
Bion
Carbendazim + Bion
Mean no. healthy shoots
% of each rhizome that
was healthy
79.4
89.8
80.0
94.5
1.00
2.44
1.44
5.22
LSD (P=0.05)
1.96
* ns= data are not significantly different
32
ns*
Table 24. The effect of Bion applied as a seed dip and /or foliar sprays on rhizome rot caused by Foz. All
seed pieces were first dipped in carbendazim. Values for means are from 12 replicate seed pieces.
Treatments
Mean no. healthy
shoots
1. No Bion treatment
2. Bion dip
3. Bion dip + 2 Bion sprays (0.025 g/L) applied 20 days apart
7. 2 Bion sprays (0.025 g/L) applied 20 days apart
5.3
3.0
2.9
3.8
2.9
5.1
5.0
2.9
3.1
4.2
% of each
rhizome that
was healthy
11.6
55.2
43.7
25.3
55.8
10.0
32.8
40.8
61.7
31.0
LSD (P=0.05)
L9
29.19
Table 25. The effect of five Bion sprays over a 14-week period on rhizome rot caused by Foz in ginger.
Treatment
Control (carbendazim only)
Carbendazim + Bion (0.025 g/L)
Carbendazim + Bion (0.05 g/L)
No. of healthy rhizomes (out of 25)
12
25
25
% of each rhizome that was
healthy
80
100
100
Table 26. The effect of various combinations of Bion treatments on rhizome rot caused by Foz in ginger.
Treatment
Mean number of
healthy shoots
1. No treatment
2. Carbendazim
3. Carbendazim + Bion dip (0.025 g/L)
4. Carbendazim + Bion dip + 3 Bion sprays (0.05 g/L) applied 10 days apart
5. Carbendazim + 2 Bion sprays (0.05 g/L) applied 20 days apart
6. Carbendazim + 3 Bion sprays (0.05 g/L) applied 10 days apart
7. Carbendazim + 3 Bion sprays (0.05 g/L) applied 10 days apart + 1 Bion
spray 3 weeks after adding Foz to soil
LSD (P=0.05)
2.65
3.27
3.41
3.07
3.87
3.35
4.42
not significant
% of each
rhizome that
was healthy
47.9
56.4
57.9
63.1
61.9
66.2
67.1
not significant
Discussion
The apphcation of Bion as a seed dip or foliar spray had no significant effect on Foz rot in ginger in two of the four
experiments. The chemical seemed to have its greatest impact when sprays were applied over a long period (e.g.
three and two sprays 10 days apart interspersed with a no spray period of eight weeks), or when it was sprayed 20
rather than 10 days apart. In pot experiment 3, only a low number of seed pieces were used in the experiment and
the level of disease in the untreated plants was low. Nevertheless, none of the plants treated with either
concentration of Bion developed disease. Further testing in the field is required to confirm these results, but they
suggest that Bion has potential as a foliar spray for control of Foz. The timing of sprays (in terms of plant age and
the amount of foliage necessary to absorb the chemical) is probably crucial in determining efficacy, as it is likely to
influence the amount of the chemical assimilated by the plant, and at what stage the defense promoters are activated.
33
6. VEGETATIVE COMPATIBILITY GROUPS IN FOZ
During surveys carried out in this project, it became apparent that losses due to Fusarium rhizome rot varied
considerably from farm to farm. One possible reason for this may have been farm to farm variation in the
pathogenicity of the fungus. As no sexual stage is known for Fusarium oxysporum, heterokaryosis (mycelium with
two or more genetically different nuclei in each cell) may play an important role in the exchange of genetic material.
Isolates of Fusarium oxysporum that form heterokaryons with each other are vegetatively compatible and form a
vegetative compatibility group (VCG). Isolates of F. oxysporum can be tested for vegetative compatibility by
pairing nitrate non-utilising mutants {nit) mutants (Puhalla 1985).
Methods
Maintenance of Foz cultures and generation of nit mutants. Cultures (wild type) of Foz stored on sterile filter
paper were reconstituted on PDA + streptomycin (120 ug/mL) and then transferred to carnation leaf agar (CLA,
water agar containing dried, gamma irradiated carnation leaf). Nit mutants were generated using the techniques of
Puhalla (1985) and Correll et al. (1987). Briefly, cultures of monoconidial isolates of Foz on CLA were transferred
to half strength potato sucrose agar containing 1.5% potassium chlorate CKPSA). The plates were incubated at
25°C and chlorate-resistant mutants that emerged as fast growing sectorsfromthe restricted colonies on KPSA were
sub-cultured on to minimal medium (MM, Correll et al. 1987). Nit mutants were generated from twenty-two Foz
isolates obtained from nine farms.
Charsterisation of nit mutants. The nit mutants were assigned to phenotypic classes (nitl, nit3 or NitM) on the
basis of their growth on media containing one of three different nitrogen sources namely nitrate, nitrite or
hypoxanthine. Nit mutants were stored on MM or as dried cultures on filter paper.
Complementation testing. Mycelia from different nit mutants were placed 15 mm apart on MM plates and the
plates were incubated at 25°C in the dark and examined periodically over 2 weeks. Vegetatively compatible nit
mutants complemented one another by forming a heterokaryon. This was easily visible as a line of dense aerial
growth of mycelium where hyphae of two sparsely growing colonies came into contact and anastomosed. The NitM
mutants were paired with themselves and with all other NitM mutants and with nitl mutants from all other isolates.
Results
Three or 4 nit mutants were recovered for each of the 22 wild type Foz isolates. The majority of mutants were nitl.
Based on complementation tests all of the isolates were grouped in the single VCG group 0461.
Discussion
Twenty-two isolates of Foz collected from nine farms were grouped in VCG 0461. This suggests that the population
of Foz within the Queensland ginger industry is relatively homogeneous at a genetic level. Since planting material
is frequently exchanged amongst farms, it is quite possible that a single strain of Foz has been introduced to most
farms over a period of years. The first introduction of Foz to Queensland is likely to have occurred in the 1930's
and has probably persisted since that time. A subsequent introduction occurred in 1954 via infected rhizomes from
China (Teakle 1965) but this disease was apparently eliminated soon after it was introduced.
34
7. SUPPRESSION OF FUSARIUM IN GINGER-GROWING SOILS
Many of the areas that are cultivated to ginger are cropped every year. These soils are heavily
contaminated with Foz because of the ginger monoculture. However, there are few farms that are cropped
to ginger once in 3-4 years, with cover crops being grown in the intervening years. These farms appear to
have less of a problem with Foz, perhaps because the soils are more suppressive to this pathogen. The
following report describes experiments that were done to test this hypothesis. Because there was a lack of
Foz-free ginger planting material and also because of high background populations of Foz (which would
have complicated interpretation of results), an alternative test had to be developed to assess
suppressiveness. A protocol involving the cotton pathogen Fusarium oxysporum f.sp. vasinfectum was
therefore used.
Methods
Experiment 1. Soils from ginger farms with different cropping histories were collected. Soils Jl and J2
were similar sandy loams from adjacent fields on the same farm. Jl had been under pasture for four years
at the time of sampling whereas J2 had been under pasture for 3 years and was planted to ginger in the
fourth year. Soil WAL was also a sandy loam that had been cropped continuously to ginger for several
years. Half of the WAL soil was autoclaved for 2 hours at 12PC, after which soil was potted up in 800mL
lots and allowed to stand for 3 weeks before using. Jl and J2 were not autoclaved because of the large
amounts of organic matter in the soils.
F oxysporum f.sp. vasinfectum (isolates 24500, 24595 and 24596 supplied by Dr. Natalie Moore,
Queensland Farming Systems Institute) were cultured on potato dextrose agar (PDA) for 1 week, and
spores were harvested in sterile water and filtered through 3 layers of muslin. The spore suspension was
serially diluted and the suspensions were thoroughly mixed into 800 g of soil per pot to give final
concentrations of 0, 103, 104, 105, 106 spores /g soil. Pasteurised peat-sand and autoclaved test soil from
WAL were used as controls. To confirm the pathogenicity of the inoculum, the root systems of 1-week-old
cotton seedlings (cv. Siokra 1-4) were dipped in a suspension of 106 fungal spores /mL for 5 min and then
planted in potting mix. All pots were placed in a glasshouse at ambient temperature (mean 18.8°C) for 1
week after which 8 cotton seeds (cv. Siokra 1-4) were sown in each pot. Before seeds were sown, all
treatments containing 10° and 105 spores /g were sampled and the number of propagules of Fusarium
oxysporum /g of soil was determined on Komada's selective medium (Komada 1975) using dilution plating.
Once seedlings had emerged, they were thinned to 5 per pot. Pots were arranged in a completely
randomised design and watered daily. The experiment was harvested after 8 weeks. Plants were rated for
severity of wilting using the following rating scheme: 0, plant healthy; 1, cotyledons only wilted; 2, <50%
of true leaves wilted; 3, > 50-90% of true leaves wilted; 4, all leaves wilted, plant dead. Plant height was
also measured (from the root/shoot interface to the base of the tenninal bud). Each plant was then split and
the % vascular staining was noted. Isolations were made from tissue with vascular staining on to PDA
containing streptomycin (120 ug /mL). Data were statistically compared using ANOVA and LSD.
Experiment 2. A second set of soils was tested under similar conditions as above. Soils WAL (sandy
loam), TEMP (red clay), FOR (sandy clay loam) and EVE (clay) had previously been cultivated to ginger
for at least 3 years. Soil MOG was a clay loam from virgin ground. The only control used was potting
mix. Disease assessment was carried out as for experiment 1.
Results
Experiment 1. Fusarium oxysporum propagules were detected in all of the treatments that were inoculated
with 105 spores /g soil. The population levels rangedfrom2.2 xl04to 4.8 xlO4 /g. No Fusarium oxysporum
propagules were detected in the non-inoculated PM or non-inoculated autoclaved treatment. Background
levels of Fusarium propagules in the non-autoclaved soils with no added Fov were less than 1.5 xl03/g.
35
The cotton seedlings that were root-dipped in the F. oxysporum f.sp. vasinfectum spore suspension
developed wilt symptoms in about 3 weeks, confirming that the inoculum was pathogenic.
Disease levels in cotton plants (expressed as % vascular staining or mean wilt rating) increased as inoculum
density of F. oxysporum f.sp. vasinfectum increased in all the non-autoclaved soils (WAL, Jl and J2) and
the potting mix (PM) (Figure 3 and Figure 4). At fungal inoculum densities of 104, 105 and 106 spores /g,
% vascular staining and mean wilt rating were significantly higher in plants growing in PM than plants
growing in the non-autoclaved soils (Tables 27-30). There was little disease in plants growing in the
autoclaved soils inoculated with F. oxysporum f.sp. vasinfectum (Tables 27 and 28). Plant height was
noticeably reduced in plants growing in potting mix at the highest inoculum density of F. oxysporum f.sp.
vasinfectum, when compared to plants in mix with no spores or 103 or 104 spores per g (data not shown).
There was no corresponding reduction in plant height in the autoclaved or non-autoclaved soils. F.
oxysporum f.sp. vasinfectum was isolated from a sample of plants with vascular staining.
120
a 100
£
1
80
60
3
U
-•-J2
-A-WF
-•-PM
40
20
H I — , — |^t^^-r—
2
3
4
Inoculum density of Fov
Figure 3. Mean % vascular staining in cotton plants growing in different soils Jl, J2, WF and PM
(potting mix) inoculated with F. oxysporum f.sp. vasinfectum. Inoculum densities 1,2, 3,4 and 5 are
respectively 10°, 103,104,10s, and 10 propagules of F. oxysporum f.sp. vasinfectum added /g soil
1.6
1.4
•
J
—h~\NF
1.2
11
-•—PM
2 0.8
|
0.6
0.4
0.2
0
-HN55* ,
•
i
*
1
2
3
4
5
Figure 4. Mean wilt rating for cotton plants growing in different soils Jl, J2, WF and PM (potting
mix) inoculated with F. oxysporum f.sp. vasinfectu. Inoculum densities 1, 2,3,4 and 5 are respectively
10°, 103,104,105, and 106 propagules of F. oxysporum f.sp. vasinfectum added /g soil
36
Table 27. Results of factorial analyses (ANOVA) (interaction table for % vascular staining x
inoculum density of F. oxysporum f.sp. vasinfectum) for potting mix and soil WAL
Inoculum density
105 spores /g
106 spores /g
LSD (P=0.05)
% Vascular staining
WAL non-autoclaved WAL -autoclaved
3
11.6
11.6
51.0
Potting
mix
38.5
100
23.12
Table 28. Results of factorial analyses (ANOVA) (interaction table for plant wilt rating x inoculum
density of F. oxysporum f.sp. vasinfectum) for potting mix and soil WAL.
Inoculum density
105 spores /g
106 spores /g
LSD (P=0.05)
WAL nonautoclaved
0.13
0.56
Plant wilt rating
WAL -autoclaved
0.04
0.04
0.30
Potting mix
0.48
1.52
Table 29. Results of factorial analyses (ANOVA) (interaction table for % vascular staining x
inoculum density of F. oxysporum f.sp. vasinfectum) for potting mix and soil J l and J2.
% Vascular staining
Inoculum density
104 spores/g
105 spores/g
106 spores /g
LSD (P=0.05)
Jl (nonautoclaved)
9.0
8.0
47.0
J2 (nonautoclaved)
19.0
12.0
45.0
24.37
Potting mix
6.0
38.5
100.0
Table 30. Results of factorial analyses (ANOVA) (interaction table for mean wilt rating x inoculum
density of F. oxysporum f.sp. vasinfectum) for potting mix and soil Jl and J2.
Mean wilt rating
Inoculum density
105 spores /g
106 spores /g
LSD (P=0.05)
Jl (nonautoclaved)
0.08
0.60
J2 (nonautoclaved)
0.12
0.60
0.59
Potting
mix
0.48
1.52
Experiment 2. The number of Fusarium propagules in the potting mix treatments 10° and 105 were
respectively 0 and 4.5 xlO5. Fusarium propagule numbers were not determined in any other treatments. In
this experiment, the PM control gave different results to those in thefirstexperiment, as plants had the least
37
number of diseased plants of all the soils (Figure 5). Therefore, data were not compared statistically.
Three soils, TEMP, EVE and FOR had a higher number of diseased plants than soils WAL and MOG.
O)
c
1
2
3
4
Figure 5. Mean % vascular staining in cotton plants growing in different soils WAL, MOG, FOR,
EVE, TEMP and PM (potting mix) inoculated with F. oxysporum f.sp. vasinfectum. Inoculum
densities 1, 2,3,4 and 5 are respectively 10°, 103,104,10s, and 106propagules of F. oxysporum f.sp.
vasinfectum added /g of soil.
Discussion
The reason for doing these experiments was to determine whether some ginger growing soils were
suppressive to Fusarium oxysporum, as there are examples in the literature of soils that are suppressive to
this pathogen (Cook and Baker 1983). In experiment 1, soil Jl had been in pasture for 4 years after the last
ginger crop and J2 had one crop of ginger following 3 years of pasture. In contrast, soil WAL had been
continuously cropped to ginger for several years. Plants in all three soils developed similar levels of
disease, suggesting that Jl and J2 were no more suppressive than WAL.
Although there were some differences in disease severity in cotton plants between soils in experiment 2,
disease levels were relatively high in all soils. These soils had been cultivated to ginger for at least 3
consecutive years and showed no signs of suppressiveness to Fusarium.
In spite of the difficulty in establishing a consistent positive control, this study showed that none of the
soils were suppressive to Fusarium oxysporum. Therefore, the low incidence of disease due to Foz in
ginger on farms with long rotations is most likely due to lower levels of Foz propagules in soil rather than
to microbial suppression. When ginger is grown once every 3-4 years, the buildup of Foz inoculum is
likely to be minimised.
38
8. CONCLUSIONS
The causes of poor emergence of ginger
The experimental results in this report provide overwhelming evidence to suggest that Foz is the primary
cause of seed emergence problems in the Queensland ginger industry.
•
•
•
•
•
•
•
•
•
•
•
A survey of potential seed crops showed that dead and yellowing shoots were commonly found in the
field from about the time of early harvest. Rhizomes from these plants usually showed discolouration
and /or disease, and Foz was consistently isolated from these rhizomes.
Surveys 6-8 weeks after planting fields with seed pieces and isolations from diseased ginger showed
that Foz was present on all farms. E, chrysanthemi was not always detected.
If E. chrysanthemi was present it was nearly always associated with Foz. When present on its own,
pathogenicity tests showed that the bacterium was able to rot seed pieces only when conditions were
conducive to the pathogen (e.g. in saturated soil).
Pythium was isolated from some farms, but none of the isolates rotted ginger in subsequent
pathogenicity tests.
Rhizome rot due to Foz was present in many fields at the time seed ginger was being harvested.
After cutting and dipping in fungicide, Foz was frequently isolated from stored ginger.
In situations where poor emergence was observed in grower's seed, Foz was the most commonly
isolated pathogen. Erwinia was sometimes present, but was not isolated from some of the severely
infected sites.
Ginger examined carefully and presumed to be "clean" could still have enough Foz infections to
prevent germination.
Pot andfieldexperiments with various chemical treatments showed that carbendazim and benomyl had
the greatest effect in reducing diseases of seed pieces. This group of fungicides affects Fusarium but
not Erwinia and Pythium.
Metalaxyl, which is only likely to have activity against Pythium, had no effect in any of the trials.
Copper (a broad-spectrum fungicide and a bactericide) sometimes reduced disease. Copper hydroxide
reduced disease in two out of three trials and copper oxychloride gave some control in three out of five
trials.
The main source of the Foz that causes poor emergence appears to be the systemic infections which
develop in rhizomes during the period from February to August. The following evidence supports this
conclusion.
•
•
•
•
Emergence can be poor when apparently clean seed is planted into pasteurised potting mix.
The source of seed and its disease status has a much greater effect on seed emergence than the
cropping history or the disease levels in the soil into which seed is planted.
When seed is obtained from heavily infected fields, fungicide treatment or appropriate storage
conditions do little to reduce disease.
If seed is relatively free of Foz, germination levels of 50-60% can be achieved, even under poor
storage conditions.
Nevertheless, storage conditions do affect seed piece emergence.
•
•
If seed ginger is stored at high temperatures or moisture levels, seed piece emergence problems will
increase.
Increasing the length of time ginger is stored after washing but prior to cutting and fungicide treatment
increases seed germination problems.
Mechanical damage during harvest and washing also appears to be a factor affecting emergence. Most
growers' seed has a high proportion of damaged buds (i.e. 50-100% of seed pieces have some cuts and
39
abrasions). It may be significant that many Canton seed pieces that fail to germinate do not rot in the first
month or two after planting. Buds become blackened and fail to grow, possibly because Foz has infected
the damaged bud tissue.
The following working hypothesis has been developed to explain the seed germination problem.
1. Ginger to be used for seed production becomes infected with Foz early in the life of the crop. Initial
infection occurs largely from infected seed pieces, with the fungus first moving into the 'neck'.
Wilting, yellowing, necrosis and death of leaves on shoots originating from the 'neck' is the first
evidence that infection has occurred.
2. By about early harvest, Foz infections have started to spread to the parts of the rhizome adjacent to the
neck. Discolouration of the rhizome begins to show and a greater number of yellowing shoots are
observed. At this stage, infection can also occur from the soil and this mode of infection is
exacerbated by nematode damage and wet conditions.
3. During the period from March to August an exponential increase in Foz occurs. Infection spreads
through the rhizome and badly infected segments begin to rot. However, segments on the same
rhizome that do not show any obvious signs of disease may still be infected, as microconidia of Foz
can move in the vascular system to other parts of the rhizome.
4. When seed ginger is harvested, Foz will always be present, but the level of infection will depend on the
level of disease in the planting material from the previous year and the previous disease history of the
field.
5. The fungus will be spread during harvesting and seed preparation operations, contaminating the
surfaces of non-infected rhizomes.
6. Provided ginger is dipped in fungicide within a few hours of harvest, Foz that is contaminating the
surface of the seed will be significantly reduced. However, the fungicide treatment will have little
impact on systemic infections.
7. During storage, fungus that is present within the seed may grow to the surface and sporulate. This
process produces large amounts of fungal inoculum that can spread and cause infection during the
planting operation, particularly if the ginger is damaged.
8. Once planted, the level of surface and systemic infections and the environmental conditions determine
the fate of the seed piece.
- Seed that is heavily infected (particularly if the infection is systemic) will rot prior to emergence,
or developing buds may be killed.
- Seed that is moderately infected may emerge but shoots will soon become yellow and die.
- High moisture will exacerbate the seed rot problem and will also increase the role of Erwinia in
the disease complex.
- Seed that is not heavily infected with Foz will germinate and grow normally, but the fungus will
eventually move into the developing rhizome and initiate infection in the new crop.
Reasons that seed germination problems are increasing
Poor seed piece emergence is no more than a severe manifestation of rhizome rotting caused by Foz. Since
Foz has been present in the ginger industry for many years, it is possible that rotting of seed pieces has
always occurred but remained undetected because losses were relatively low (i.e. < 20% diseased seed
pieces). There is no single reason for the increase in disease severity in recent years, but the following
factors appear to be involved
» The increasing proportion of market ginger. Levels of Foz increase exponentially as the age of a
ginger planting increases. Thus high populations of Foz are left in soil when crops are kept for more
than 12 months. These Foz propagules may not affect emergence when clean seed is planted, but they
ensure that seed produced in thatfieldwill be heavily infected.
• More frequent use of land for ginger. Economic pressures are forcing many growers to crop more
frequently, with most ginger land now being cropped nearly every year. This increases Foz to
epidemic levels and ensures that high population densities are maintained. Once a gingerfieldis
40
infested, it probably takes 4-5 years of a non-host crop for the inoculum density to decline to levels
where seed production could even be considered.
• The increasing area that is planted to Canton, a variety that has relatively few knobs per seed piece.
Canton appears more likely to rot before emergence than Queensland ginger, perhaps because it has
fewer potential new shoots in reserve when Foz causes damage to some of the knobs.
» A decrease in fungicide dipping time. The switch to mechanised dipping operations has meant that
ginger is dipped for only 1-2 minutes, rather than the recommended time of five minutes.
» Increased mechanisation, which means that there is more damage to seed from abrasion during the
digging and planting operations. Cuts and indentations provide entry points for Foz and allow surface
contaminants to be introduced deeper into the rhizome.
« Introduction of mechanised washing of ginger, which damages rhizomes and sometimes involves
contamination of rhizomes with re-circulated water.
• Heavier infestations of root-knot nematode (due to more frequent cultivation of ginger and less crop
rotation), which exacerbate Foz problems.
• An increase in the amount of infested ginger left in thefieldafter harvest. This increases the amount
of inoculum carried over to the next crop.
• A reduction in the suppressiveness of soils to Foz. When high populations of soil microorganisms are
present in soil, they compete with Foz and tend to reduce its severity. Lack of rotation and reduced
inputs of organic matter deplete the soil biota and tend to produce soils that are more conducive to Foz.
• The problems of poor emergence in 1997 and 1998 were exacerbated by excessive rainfall. Moisture
has little impact on Foz, but infection of ginger by E. chrysanthemi is exacerbated when soil is
saturated.
41
9. RECOMMENDATIONS
Because most Queensland ginger fields are already infested with Foz and the pathogen is invariably
introduced into new land on infested planting material, seed piece rot due to Foz is likely to remain a
continuing problem. However, there are many things that the industry can do to manage the problem and
reduce losses to acceptable levels.
Modifications to current seed production/preparation
practices
Although Foz is present on all ginger farms, significant seed germination problems do not occur on all
farms. This suggests that with appropriate seed production and preparation procedures, Foz problems can
be limited to levels that do not have a major economic impact. However, most or all of the following
practices will have to be adopted if low disease levels are to be maintained.
• Seed production must be given high priority within the farm management operation, with decisions on
the location of plantings to be used for seed being made at least 18 months before seed is required.
• Control of root-knot nematode is important, as this pest damages ginger and makes it more vulnerable
to invasion by Foz and E. chrysanthemi.
• Infection by E. chrysanthemi is exacerbated by high soil moisture. Potential seed patches should
therefore be planted in well-drained sites.
• Ginger that is used for seed must be grown in fields that have never grown ginger.
• Potential seed patches must be inspected several times from March until pseudostem senescence, and
any plants showing yellowing and /or dead shoots dug up and removed from the field.
• When seed ginger is harvested, it must be sorted in the field and obviously diseased rhizomes
discarded (i.e. taken off-farm and destroyed).
• Rhizomes should be washed in clean rather than re-circulated water.
• Washed ginger must be carefully inspected and rhizomes that show any sign of discolouration or
disease must be diverted to uses other than seed. Thus only the cleanest ginger reaches the cutting
table.
• Ginger must be cut soon after rhizomes are washed.
• Cut seed should be rigorously inspected on the cutting table, with all discoloured pieces being rejected.
• High standards of hygiene must be maintained during the cutting operation
• Seed should be dipped in a registered fungicide immediately after it is cut.
• After dipping, seed should be stored in a cool, dry, well-ventilated area until it is planted.
Current evidence suggests that seed germination problems would be reduced significantly if all the above
practices were implemented on an industry-wide basis. One of the most important management practices
that should be adopted is regular inspection of fields that are to be used for seed. Inspections should be
done during March, April and May, with plants that show any signs of disease being used for factory or
market ginger. This culling process was used successfully in the clean planting material scheme that
operated prior to about 1980.
Because of the high levels of Foz in most ginger fields, the above procedures may need to be used for
several years before reasonably clean seed is obtained. Growers who currently have a chronic Foz problem
will have to be particularly attentive to detail.
Introduce a clean seed scheme based on tissue-cultured
ginger
The above modifications to current seed production and preparation practices will reduce seed germination
losses and ensure that late-harvest losses due to rhizome rot do not reach unacceptable levels. However,
Foz will still occur and there will always be a risk that inattention to detail at any stage of the seed
production process will cause the disease to reappear at unacceptable levels. A clean seed scheme would
therefore be a far safer and more professional way for ginger growers to tackle the problem in the medium
42
to long term. Such a scheme could be established on an industry-wide basis, or set up by an individual
grower.
Tissue cultured ginger is available commercially and would form the basis of a clean seed scheme. In the
first year, this material would be grown in sterile potting mix in a nursery and the small rhizomes produced
from these plants would be the initial source of disease-free planting material. These rhizomes would be
planted into soil that had never grown ginger to produce a mother planting of disease-free material. Seed
from the mother planting would then be grown on in clean land to produce pathogen-free first generation
seed pieces. Strict hygiene would have to be employed in these plantings to ensure that Foz was not
introduced on machinery or by other means. Provided a new batch of rhizomesfromtissue-cultured ginger
is planted every year, this process would provide a continuing supply of clean ginger seed pieces that could
be used for further multiplication.
Extension material
The above recommendations were included in a technical manual entitled 'Improving emergence in ginger
planting material' that was distributed to all ginger growers in May 2000.
10. FURTHER RESEARCH
Results of glasshouse experiments with Bion (section 5) showed that plants sprayed several times over a 3month period were not infected by Foz, This suggests that this plant-defense-promoting chemical may
have potential for reducing Foz infection in ginger late in the growing season. Further work is needed to
confirm this result, but if this is successful, it may be possible to spray seed patches with Bion and reduce
levels of Foz in the following season's planting material. Such a process is never likely to be an alternative
to the control options discussed above, but it could possibly be successful if used in conjunction with
appropriate management practices.
43
11. LITERATURE CITED
Barnett, H. L. and Hunter, B. B. (1998). Illustrated Genera of Imperfect Fungi. Fourth Edition. APS Press,
St Paul MN.
Burgess. L. W., Summerell, B. A., Bullock. S., Gott, K. P. and Backhouse, D. (1994). Laboratory Manual
for Fusarium Research. University of Sydney.
Cook, R.J. and Baker, K. F. (1983). The Nature and Practice of Biological Control of Plant Pathogens.
APS, St. Paul, MN.
Correll, J. C, Klittich. C. J. R. and Leslie, J. F. (1987). Nitrate non-utilising mutants of Fusarium
oxysporum and their use in vegetative compatibility tests. Phytopathology 77: 1640-1646.
Dake, G. N. and Edison, S. (1989). Association of pathogens with rhizome rot of ginger in Kerala. Indian
Phytopathology. 42: 116-119.
Fahy, P. C. and Hayward. A. C. (1983). Media and methods for isolation and diagnostic tests. In. Plant
Bacterial Diseases: A Diagnostic Guide. Eds. P C Fahy and G J Persley. Academic Press.
Grech, N.M. and Swarts, D. H. 1990. Post-harvest application of fungicides for control of fungal decay of
ginger rhizomes stored under simulated low-temperature shipping conditions. Phytophylactica 22:457-458.
Howell, C. R., Beier, R. C. and Stipanovic , R. D. (1988). Production of ammonia by Enterobacter cloacae
and its possible role in the biological control of Pythium pre-emergence damping-off by the bacterium.
PhytopatologylZ: 1075-1078.
Komada, H. (1975). Development of a selective medium for quantitative isolation of Fusarium oxysporum
from natural soil. Review of Plant Protection Research 8: 114-125.
Mishra, B. and Rath, G. C. (1988). Geotrichum rot of stored ginger. Indian Journal of Mycology and Plant
Pathology. 18: 213.
Nelson, E. B. (1988). Biological control of Pythium seed rot and pre-emergence damping-off of cotton with
Enterobacter cloacae and Erwinia herbicola applied as seed treatments. Plant Disease 72: 140-142.
Persley. D. (1994) (ed). Diseases of Vegetable Crops. Department of Primary Industries Queensland.
Pegg. K. G., Moffet, M. L. and Colbran, R. C. (1974). Diseases of ginger in Queensland. Advisory Leaflet
No. 1284. Division of Plant Industry, Department of Primary Industries.
Puhalla, J. E. (1985). Classification of strains of Fusarium oxysporum on the basis of vegetative
compatibility. Canadian Journal of Botany 63: 179-183.
Teakle. D. S. (1965). Fusarium rhizome rot of ginger in Queensland. Queensland Journal of Agricultural
and Animal Sciences. 22:265-272.
Trujillo, E. E. (1964). Diseases of ginger (Zingiber officinale) in Hawaii. Circular 62, Hawaii Agricultural
Experiment Station, University of Hawaii.
44

Overcoming seed quality problems in the ginger industry

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