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Hopkins
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A Review of Moulding of Forest Nursery Seedlings in Cold Storage
By
John C. Hopkins
Canadian Forestry Service
Pacific Forest Research Centre
Victoria, B.C.
Report BCX12B
Department of the Environment
November, 1975
Revue de la litterature relative a ta moisissure des semis d'abres
forestiers entreposes au froid, avec des paragraphes distincts sur I'etiologie
et les degats de cette infection, les facteurs determinants de sa presence et
de sa gravite et les methodes de repression de la moisissure. Description du
systeme employe en Colombie Britannique pour l'entreposage des semis
et examen des moyens de lutte susceptibles d'y etre appliques contre la
molsissure. Elucidation des imperatifs de la recherche dans ce domaine.
. 1.
ABSTRACT
Literature on moulding of forest nursery seedlings in cold storage
is reviewed, with separate sections on etiology and damage, factors influencing
the occurrence and severity of moulding, and control procedures. The system
for storage in British Columbia is described and the prospects for mould
control are examined for probable applicability there. Research needs are
identified.
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INTRODUCTION
Cold storage of forest nursery seedlings has
been used in many countries to maintain stock
in a dormant state until required for spring
or early summer planting (28). Methods for
cold storage have been known for many years
(30) but, with improved techniques. interest
has increased rapidly in recent years. Refrigerated storage is now used extensively in
British Columbia for bare root and container
stock, with facilities at each nursery.
Cold storage permits stock to be kept dormant
in spring until sites are ready for planting 1561.
An adverse effect on survival of lifting and
outplanting after buds have begun to swell
has been reported (25). For this reason alone,
cold storage is invaluable for spring planting
in British Columbia with its extensive mountainous terrain. However, cold storage of
seedlings also confers important advantages in
terms of seedling quality and in the efficiency
of nursery operations.ltean provide an extension of the seasonal period for maximum root
regeneration potential, a factor considered
important in seedling establishment (25). It
permits lifting and sorting to be carried out
during otherwise slack periods (28) and it
permits large numbers of seedlings to be
shipped within a short period and at very
short notice (13).
A serious threat to the quality of stock maintained in storage arises from the growth of
moulds on the seedlings. In B.C., a long storage period, in some instances extending from
mid·November to mid-June, is likely to
increase the importance of moulding. In
1973-74 in B.C., approximately 6.5 million seedlings of white spruce (Picea glauca (Moench)
Voss), western hemlock (Tsuga hcterophylla
(Raf.) Sarg.) and Douglas-fir (Pseudotsuga
menziesii IMirb.) Franco) were found to have
extensive moulding. However, experience
suggests that the damage levels fluctuate from
year to year.
This report reviews the information on the
etiology of moulding of forest nursery seedlings in cold storage, factors influencing the
occurrence and extent of moulding in storage,
the resulting damage, and control methods.
Emphasis is placed on items considered to be
most relevant to conditions in B.C.
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STORAGE SYSTEMS
Cold storage arrangements and facilities have
evolved from the earliest forms in which some
cover is provided over winter for heeled in
plants to complex arrangements for ensuring
precisely controlled temperature and humidity
conditions (381. A recent survey of facilities
in forest nurseries throughout the United
States and Canada indicates how widespread
the use of refrigerated storage has become
1271_
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Alternative types of refrigerated storage have
been reviewed by several authors (13, 28, 53)
and fall into two main categories, i.e., direct
or indirect refrigeration.
Direct cooling facilities, the most widely used
for forest nursery storage in North America,
are less expensive to construct and provide
some operational advantages over indirect
systems. However, the systems in use dry the
air passing over the cooling coils. The resulting
desiccation of plants has been reported to
lower survival rates substantially (36), and
necessitates storage in waterproof bags or
boxes, often incorporating polythene (28).
The conditions of high humidity created
within these enclosures can encourage mould
growth on the seedlings (131.
Indirect cooling systems employ secondary
refrigeration media, thus permitting close
temperature control and the maintenance of
small temperature differentials between the
cooling surfaces and the ambient air. This
low differential can avoid substantial drying
of the air and permits, with automatic humi·
dification, maintenance of 90-95% relative
humidities and open storage of seedlings
•
-3 without serious desiccation. Open storage
permits illumination of the stored plants.
In B.C., direct cooling is provided at each
nursery in special storage rooms with insulated walls. Stock to be stored are packaged
for storage from mid-November to March and
occasionally into April. Plants are packaged
in bundles of 50 and tied with twine or plastic film. Container stock is kept in boxes,
while bare-root stock is placed in waterproof
bags before placement in boxes_ Container
stock is normally packed vertically, but bareroot stock may be packed horizontally or
vertically. Thermostat settings have varied
but, currently, most are set at approximately
(JOC.
Potential sources of temperature variability
within a system with direct refrigeration are
numerous. The variability of a simple thermostatic control coupled with a simple on-011
response system has been well explored (7).
A large temperature differential can develop
behveen ambient air and the coils. Uneven
air circulation, as influenced by the amount
stored, can be expected to add to the variability. Opening of doors can influence temperature and a curtain of air involving a rapid
vertical flow at a doorway has been used to
minimize this (13). Fail safe devices incorporating a thermostat set slightly below the
operati ngtemperature have been recommended
(63) and alarm systems may warn of control
failures.
A major source of temperature variability
may develop as a result of temperature elevation within boxes (28). Higher box temperatures have been attributed to seedling respiration (44). An increase of 6.6-12.2 0 C above
ambient has been reported (60).
ETIOLOGY AND DAMAGE
ETIOLOGY
Numerous fungi, mostly known soil inhabitants 1281. are associated with moulding of
stored tree seedlings. Unfortunately, most
reports provide no information on the fre·
quency of occurrence and in only a few
instances have inoculations been used to
provide data on the nature of the association.
Thirty-seven species of fungi were isolated from
moulded foliage and buds of cold·stored
white spruce ~Picea 91auca var. albertiana
(S. Brown) Sarg.) and lodgepole pine (Pinus
contorta vaT. latifo/ia Engelm.) seedlings (261.
Inoculations with 10 of the most prevalent
fungi provided evidenceof their pathogenicity.
These fungi were Epicaccum purpuraseens
Ehrenb. ex Schlecht., Fusarium oxysporum
Schlect. ex Fries, Phama g/omerata (Cda.)
Wr. and Hochapf., Penicillium eye/opium
Wrestling, a Phoma sp., Pythium ultimum
Trow, a Fusarium sp., and three distinct
Mycelia sterilia.
An earlier study on moulding of white spruce
in storage in Saskatchewan reported isolation
of the same fungi as had been isolated from a
snow mould in the nursery 1671. Isolations
yielded species of Pesta/otia, Hormodendron,
Fusarium, Phoma and Epieoccum. Extensive
storage moulding was associated with a
Penicillium. Mycelial inoculations on 2-year-old
seedings proved pathogenicity of the Pestalotia,
identified tentatively as P. hartigii Tubeuf.,
on white spruce and on jack pine (Pinus
bclflksiana Lamb).
Moulding of many species of plants in cold
storage by Botrytis cinerea (FT.) Pers. has
been reported from Europe(21) and California
157, 58). The author has observed B. cinerea
on moulded seedlings of western hemlock and
Douglas·fir in cold storage.
Isolations from stored seedlings in New York
State yielded 45 different fungal species
(45), mainly species of Botrytis, Trichoderma,
Fusarium and Penicillium.
Damage to stored coniferous seedlings in
Norway was caused by Mycelia sterilia (54).
In Sweden, Phacidium infestans, Karst, a
common snow mould there, caused substan·
.4.
tial damage in storage (10).
from conifers, might defer damage extension
until conditions become favorable (40).
A Cylindrocarpon sp. was associated with
moulding of seedlings stored in the United
Kingdom (13).
Root damage in stored Douglas-fir in Oregon
has been attributed to a Rhizocton;a sp. (16).
Moulding of roots of stored Manitoba maple
(Acer negundo L.) was associated with
species of Penicillium and Fusarium (66).
These reports indicate that moulding is
caused by fungi which belong to genera
that are common soil inhabitants. However,
bacterial damage may precede fungal moulding
(44).
DAMAGE
O.t
Mortality during storage has occurred as a
result of moulding. Death of 10·40% of stock
in some U.K. trials has been attributed to
moulding (1). Moulded Manitoba maple seedlings were killed in storage (661. Death of
western hemlock seedlings in storage from
damage by B. cinerea has been observed by
the author.
Large amounts of stored stock in Ontario have
been discarded as a result of moulding (431It may be preferable to discard moulded stock
rather than risk poor survival, which necessitates replanting.
Survival of severely moulded white spruce
was just below 40%. while that of similar
Few studies have dealt with the nature and but healthy stock was 85% (26)' In another
significance of damage by moulds to cold trial, survival of severely moulded white
stored seedlings. Very few reports on the spruce declined frOm 97% in the control group
influence of moulds on growth and survival to 5% (26). In the same trial, survival of white
of the outplanted seedlings identified the spruce with 25-75% of the foliage moulded
was 21%, while in others with 1-24% of their
mould responsible for the damage.
foliage moulded, 63"'" survived. In a trial of
In this review cold storage damage includes lodgepole pine, survival declined from 84%
that initiated in cold storage and damage con- in the healthy group to 52% in the group
tinuing in cold storage from infections initiated with 1-24% of the foliage moulded, to 12%
during active growth oftheseedlings. Moulding in the 25·75% moulded foliage group, and to
may also continue in transit to the planting 0% in the 75·100% moulded foliage group
site from storage and during temporary field (26). However, several observations by the
storage. Damage observed in stored seedlings author of survival of partially moulded
of western hemlock was traced to stock known Douglas-fir and white spruce seedlings suggest
to have infections of 8. cinerea present before that survival may sometimes be higher than
storage (personal observation). The temper- the published data indicate. Differences in
ature relationships of some mould fungi (26) survival following moulding may be related
suggest that damage probably accelerates to the causal organism, the site of the damage,
under non-refrigerated conditions. The influ- and to the degree and type of host penetraence of exposure to wind and sun on mould tion. Establishment of Douglas-fir transplants
survival on affected seedlings is unknown. after cold storage has been shown to depend
Arrest of mould development, especially on an unknown substance for root developsuperficial mycelium, may occur under some ment originating in the needles (37). Loss of
cinerea moulded needles might reduce the supply
weather conditions, but reports of
damage within an established German forest of the root development substance.
during a wet spring (75) and the activity of
this pathogen as a snow mould suggest that Data from trials indicate that storage mould
damage may continue after outplanting. can reduce subsequent growth rates of surLatency of B. cinerea, although not reported viving plants. The mean terminal growth of
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white spruce seedlings in the year of planting
was 10.9 em (4.3") in the control group and
0.5 em (0.2"1 in the severely moulded group
1261. Similarly, the mean terminal growth of
lodgepole pine seedlings was 13.2 em (5.2"),
compared with 3.3 em (1.3") in the category
with 25-74% of the foliage moulded (26).
Damage to terminal buds of white spruce
and lodgepole pine seedlings, following
inoculation with 10 species of moulds,
can occur and could explain the reductions
in terminal growth (26). Needle loss and
needle damage by moulds can be expected
to influence photosynthetic capacity and
hence reduce growth in the year following
planting. Trials in Ontario indicated that
storage of white spruce for 2-8 weeks led
to a reduction in height increment in the
first and second years following planting,
but no effect was detectable in the third
year (50).
FACTORS INFLUENCING OCCURRENCE
ANO SEVERITY OF MOULDING
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INOCULUM
Soil particles deposited on stock during
lifting, sorting and packing helve been
reported to constitute the common source
of inoculum for moulding (26). This concept
is supported by evidence that the moulding
organisms identified so far are soi I inhabitants
(28). The existence of other sources of
inoculum is implicit in control recommendations to avoid storing seedling already
infected with needle casts, snow blight,
grey mould or root rots (27). Infected stock
may constitute the major inoculum source
for container·grown seedlings since there are
considerably fewer soil particles on the
foliage, compared with bare root stock.
Observations by the author of severe mou Iding in some boxes of container-grown stock
believed to have scattered infections of
8. cinerea at placement in storage lends
support to this interpretation. Packing
materials, particularly peat, have also been
implicated as a source of mould organisms
1161.
No firm information is available on the
relative importance of spores and of mycelium in the spread of mould within storage.
Mycelia observed ramifying around twine
used to tie bundles may be important in
this regard.
Few studies have dealt specifically with the
types, sources and distribution of inoculum
responsible for moulding in storage but
information exists on inoculum formation
and distribution of one mould, B. cinerea.
Botrytis cinerea is found in all temperate
regions (58) and has been classified as a
saprophyte which acts frequently as a
primary colonizer of plant material (31).
In the United Kingdom, it commonly occurs
on the lowermost needles in dense beds (49).
In B.C., it has been reported as damaging to
lower needles of bare·root stock of Douglasfir (11, 48). This fungus spreads primarily
by conidia, (conidiospores) although ascospores do occur (49). Conidia are released
as a result of changes in relative humidity,
with subsequent dispersal by air currents
or rain splash droplets (33).
HOST FACTORS
Comparatively little is known about variation in host susceptibility to moulding, but
there are reports linking susceptibility with
host species, tissue maturity or degree of
dormancy, nutrition and damage (13, 44,
45).
In Alberta, lodgepole pine was consistently
less susceptible to moulding than white
spruce stored under similar conditions (26).
Operational experience in the New York
area indicated that eastern white pine (Pinus
strobus L.), white spruce, and balsam fir
(Abies balsamea (l.) Mill.) were the species
most prone to moulding (451.
Mahlstede and Fletcher (38) suggested that
a seasonal fluctuation in susceptibility of
seedlings to rnoulding in storage occurs,
. 6·
which is linked in some way to the hardening
off process; with fully dormant seedlings as
the least susceptible. Cultural treatments to
hasten hardening off of several deciduous
shrubs increased their resistance to 8. cinerea
(21). Also, treatment of sugi seedlings
(Cryptomeria japonica D. Don.) with maleic
hydrazide, a growth inhibitor, reduced
damage by 8. cinerea. acting as a snow
mould (55). A seasonal change in host
susceptibility, if valid, could help explain
increased moulding of several coniferous
species found when early fall lifting and
storage was used in the U.K. (1).
Nutrient regimes with high nitrogen and low
potassium levels predispose several deciduous
species to attack by B. cinerea (21). Increased
susceptibility to infection by 8. cinerea of
the more succulent individuals of Douglas-fir
stock in B.C. may involve this mechanism
(111. Potassium deficient sugi seedlings in
Japan (55) and Scots pine (Pinus sylvestris
L.) seedlings in Finland (351 were more
susceptible to snow mould. However, nitrogen
levels in Finland had no influence on snow
mould damJge (35), Studies of several plant
diseases in agricultural crops have related
high nitrogen levels to increased susceptibility
to disease, whereas high potassium and
phosphorus levels tended to decrease it
(221.
Localized wounding of stem and foliar tissues
by insects, toxic chemicals, or frosts may
create infection courts for 8. cinerea,
although healthy tissue may be attacked
directly (48). The damaged tissues colonized
by the fungus may constitute a food source
and allow a facultative parasite such as
8. cinerea to invade adjoining healthy tissues.
STORAGE ENVIRONMENT
The temperature at which stock is stored
and the moisture relationship of that stock
in storage are considered to be major factors
influencing the occurrence and severity of
moulding. Information on the temperature-
growth relationships of the mould species
characteristic of an area, wi th consideration
for iso[Jte variability,can provide information
directly utilizable for control.
The highest growth rates of the 10 fungi most
frequently isolated from mouldy seedlings in
Alberta occurred at bet\Veen 20 and 25 0 C,
and the rates 'Nere sharply reduced between
15 and 5 0 C (26). All but two of these
species grew at OOC, although slowly. In a
study of 8. cinerea (21), growth occurred at
-2°C on agar plates kept at 98% relative
humidity, although it was not measurable
for 20 days after inoculation. Growth of
these isolates of 8. cinerea aher 40 days
was 1.0 cm at ·2 0 C, 2.5 cm at 10 C, 4.0 cm
at 2 0 C and 7.5 cm at 40C. A similar study
of conidial germination indicated that 40
days elapsed at 10 C before germination
reached 88%, while 90% occurred after 22
days at 2 0 C and 87% occurred aher 16 days
at 40C (211. In another study of 8. cinerea
(65) on agar, similar dJta were obtained but
the curve extrapolated to indicate zero
growth at ·l o C. However, 8. cinerea is a
highly variable species (41).
Few studies have dealt with the significance
and role of stock moisture status or condition
in the development of moulding, although
operational experience suggests that free
moisture can playa major role in the development of extensive moulding (18). The
requirement of free moisture for the initiation
of moulding was demonstrated by inoculations in Alberta (261. However, it is uncertain
how closely the experimental conditions
simulate operational conditions, and observations by the author of some stock stored
'Net at a temperature above freezing, yet
without visible moulding, raises questions
as to the assumption. Nevertheless, the
operational experiences, together with the
requirement of free moisture for spore
germinJtion among most fungi (15), does
suggest that the occurrence of free moisture
regulJtcs the initiation of moulding in many
instances. Information is lacking on the
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influence of free moisture and of high relative
humidity values on mycelial development of
moulding species. Mycelium may be responsi·
ble for intensification within a bundle. Mycelia
of many species may depend on the existence
of high relative humidities for continued
growth. Mycelium of B. cinerea sUivived over
12 months at 95-1D<::m RH, 5-12 months at
90% RH and 2-12 months at 85% RH 1651.
CONTROL METHODS
Awidevariety of measures have been suggested
or recommended for control of moulds in
storage (281. However, few of these have
been adequately evaluated and even those
that have been found useful may have a
restricted value, depending on host species,
mould species and storage environment. The
variation in success, especially with fungicides,
indicates the value of testing under local
conditions.
was much lower at ·2°C, compared with
laC, but it was lowered only slightly more
at ·Goe (54). However, a thawing period may
be necessary to permit separation of plants
and could permit resumption of mould
development and delay shipment of stock.
It may be anticipated that optimum use of
this approach requires information on the
timing and degree of attainment of low
temperature hardiness for each species and
on the low temperature growth relationships
of the moulds_
A number of favorable reports on control
based on operational experience with low
temperatures exist. In Norway, moulds were
absent from Norway spruce stored at -5 to
-3 0 e (3), while other data from there indicate
that -2CC was sufficient to inhibit moulding
(711. Some mould developed at a to _laC
(3). A Swedish report indicated a risk of
moulding at ()OC or above (10). In Germany,
o to -20 C inhibited moulding with a variety
of plants (21). In the U.K., storage at ()O and
Control measures may be preventative or at 2 0 e gave good control of moulds provided
curative. The former approach has been foliage was dry when packed (131. In the
advocated (38), but its effectiveness may eastern U.S., stock remained free of moulds
re<luire a considerable understanding of the when storage temperatures were lowered to
specific characteristics of the mould-host -2.2 to ·1.1 0 C (44). White spruce in Alberta
interaction. A curative approach, applied to remained free of mould for 7 months when
packaged stock, would necessitate periodic maintained at -3.9 0 C (29).
sampling during storage, with the attendant
risk of non·detection until damage is consider- The favorable reports of operational use of
low storage tempertltures is in general accord
able.
with information on the effect of low temSTORAGE TEMPERATURE
peratures on the germination and growth of
those moulds that have been studied (21,26,
Low storage temperatures constitute one 651. The lowest temperature at which micro·
method of mould control that has been bitll growth is thought to occur is -1 ooe (32).
widely recommended for some host species
(28, 451. Mould control by this method Variation in hardiness at fall dormancy
involves storage at temperatures inhibitory between tree species and provenances may
to growth of the mould species yet within influence the selection of storage temperathe low temperature tolerance of the dormant, tures. In storage trials in Holland of -20 C,
or partially dormant, seedling. This approach OOC and 10 C, Douglas-fir and Corsican pine
may have the additional advantage of lessen- stored best at ()OC (461. In Germany, many
ing the respiration of the host and of preserv- species survived well aher long storage at
ing food reserves, compared to other methods -5°C, but Douglas-fir survived well only at
involving higher temperatures (44). The OOC (47). However, U.K. trials indicated
respiration rate of Norway spruce in storage that Douglas-fir lifted in early February
-8 tolerated -50 C as well as 20 C (11- Studies of
an interior provenance of Douglas-fir (34)
indicated that it stored well at -1.5°C.
storing only surface dry plants (18). Stock
was watered before shipment if excessive
drying had occurred as a result of efforts to
keep moisture values down.
Variability may occur from year to year in
the degree of hardening off reached by a
given date. Hardening off in Douglas-fir
has been shown to be influenced by photo-
period and low temperatures (68,691. Mineral
nutrition of the stock. as influenced by applications of nitrogenous and potassium fertilizers, may also influence the degree of
hardening oH reached (8). In many plants,
moisture stress enhances the development of
cold tolerance (5), although this may not be
valid in Douglas-fir (68).
Studies in Holland on optimal lifting dates for
An inter-relationship between free moisture
and temperature in influencing moulding has
been reported (21. Stock packed in a wet
condition but stored at sub·freezing temperatures remained free from mould, In the U.K.,
little moulding was detected in plants stored
at 2 °C or lower, provided they were surface
dry when placed in storage (13). Rapid moulding has been found during the thawing period
subsequent to storage at sub-freezing temperatures if free moisture was not removed (54).
In Finland, no adverse results were detected
from freezing of wet stock (74).
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cold storage led to a recommendation that
lifting of Douglas-fir be restricted to a period
from mid·January to early February, but
other species of conifers could be lifted from
early December onward (46), Information
supporting this view is provided by data that
sensitivity of roots of Douglas-fir in Oregon
to damage, and loss of root regeneration
capacity from exposure, decreased substantially between November and January 123,
24). Other studies on root regeneration capacity of this species in storage showed that
a gradual loss of potential occurred in stock
stored at 2°C, but this loss was prevented by
maintaining the roots at 150C (37). However,
operational experience in B.C. with Douglas-fir
bare-root stock lifted for storage from early
December onward in coastal nurseries appears
to have been generally favorable.
MOISTURE
The importance for mould control of preventing the occurrence of free moisture on the
surfaces of stored seedlings has been stressed
12, 18, 261. Experience in the U.K., with its
moist fall and winter climate, has been that
free moisture on the plants when placed in
storage led to moulding (13). Spin dryers and
fan heaters have been used there to dry foliage
before storage. In New York State, experience
with moulding has indicated the value of
The importance of maintaining close tempera·
ture control to avoid fluctuations resulting
in condensation onto foliage has been reported
(261. Information on the quantitative rela·
tionships involved within boxes of otherwise
dry seedlings would assist in evaluating this
approach.
In B.C., the coastal climate during the traditional lifting period of mid-November onward
through the winter is frequently wet and
storage of wet stock may be unavoidable.
Container-grown seedlings may, however, be
surface dried under shelter before placement
in storage.
Aeration holes in bags or boxes used in
enclosed storage systems has been suggested
as a method for permitting evaporation of
excess moisture (13). However, desiccation
of stock can have a serious effect on survival
after planting (36), and experimentatiQn on
the effect of hole size, number and distribution would be necessary to evaluate this
approach.
SANITATION
Sanitation procedures suggested for mould
control have been based either on minimizing
contact between inoculum and susceptible
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the protective types but with good results
from the few tests using systemic types.
Benomyl. applied as a pre-Ii hing field spray
or as a post-lifting plant dip, gave excellent
control of a storage mould of giant Sequoia
( Sequoiadendron giganteum (LindLl Bucholz)
seedlings in California (57, 58>' Data on a
test of a field spray of jack pine provided
highest survival among plants treated with
the systemic fungicide methyl thiophanate,
but several others, including benomyl and
foliage.
karathane, also improved survival; control
Exclusion from storage of seedlings that of storage moulds was considered responhave been damaged by needle casts, grey sible for the favorable result (141. Also,
mould or root rots (44) has been recom- unpublished data by the author provide
mended. although exclusion of minor levels evidence of the effectiveness of benomyl
of damage from these and other sources is in arresting moulding of stor\,;d white spruce.
likely to be impractical. Dead needles are
commonly invaded by 8. cinerea (31) and The appearance of strains of fungi tolerant
other damaged tissues may harbor this fungus to benomyl from within species initially
and permit spread into healthy tissues (49), sensitive to it, a phenomenon reported from
many parts of the world (17), could limit
Removal of moulded plants from storage has the usefulness of this fungicide for mould
also been recommended as a means of con- control. Stability of the tolerance over many
trolling storage mould 1441. This approach transfers on agar indicated a genetic basis
is likely to be more important in open storage for it (6, 42). Cross·tolerance, reported
within the benzimidazole group of systemic
arrangements than in enclosed ones.
fungicides that contains benomyl and methyl
The importance of hygiene within storage thiophanate 16. 121 indicates that methyl
chambers and the value of sterilizing agents thiophanate would be ineffective against
and of cleaning procedures to minimize benomyl tolerant strains. However, a restriccarriage of inoculum from preceding storage tion in use of benomy! in the nursery to
periods has been emphasized (13, 44). Fumi- those situations for which no alternative to
gation with methyl bromide or treatment the benzimidazole group exists, should
with chlorine bleaches has been suggested minimize the selection pressure for tolerant
(441. Fungicidal sprays such as Bordeaux forms. Although reports on the period elapsmix tu re and su Iphur sprays have also been ing between initial use of benomyl in field
suggested (38), although unsightly residues situations and the appearance of tolerant
may develop. However, the value of any strains vary betVlo'een 3 months (39) and over
sterilizing treatment is unknown. The long- 1 year (62), it seems unlikely that a single
evity of mould spores within cold chambers application of benomyl immediately prior
is unknown. Sterilizing treatments may be to storage, with subsequent shipment out
of greater value for open storage arrange- of the nursery. would lead to the appearance
ments than for enclosed ones.
of tolerant individuals of moulding fungi.
host tissues or on exclusion of inoculum
from stored material (44). Suggestions for
minimizing contact appear to involve the
assumption that soil particles distributed over
foliage during lifting and packing constitute
the common source of inoculum (261. However, it appears impractical to restrict soil
deposition on bare-root stock. Container
grown stock placed in storage appears to
be relatively free from soil particles on
FUNGICIDES
Id
19
Ie
Numerous fungicides have been tried for
control of storage moulds, some protective
and some systemic, with varying results from
Attempts to control moulding with surfaceactive fungicides have often provided negative
or inconclusive results (281. possibly because
of application aher infection, uneven coverage, loss, or of low toxicity to the species
. 10·
concerned. Poor adhesion of thiram on stored
seedlings (51) suggests that poor retention of
surface active fungicides may be responsible
for some of the negative results. Application
in dust form has been recommended, since
the high moisture levels associated with liquid
application has been reported to increase
moulding in some instances (381.
Field spray applications of chlorothalonil and
difolatan provided some protection against
grey mould of giant Sequoia in storage but
they were less effective than benomyl (571.
Pentachloronitrobenzene provided some control with conifers in storage (3, 71) and was
used successfully to combat snow mould
(521. In one trial, captan was the only fungicide that controlled moulding of stored tree
seedlings (71). A captan dust was effective
against moulding of several species of shrubs
in storage (38, 72). Lime-sulphur was effective against snow blight in the field (10).
Fumigation with tetrachloronitrobenzene has
been suggested for control of mould on
coniferous stock in storage (44).
OTHER POTENTIAL CONTROLS
Delaying lifting of stock until it is completely
dormant has been suggested as a means of
controlling moulds, on the assumption that
resistance to mou Idi ng develops concom itantly
with hardening off and dormancy (441.
Operational experience with storage of a
variety of plants also points to the importance
of this factor (38), but experimental evidence
is lacking.
A nutritional approach to mould control
based on data linking tissue susceptibility
with relatively high levels of nitrogen and
low levels of potassium (21,351, if validated
for coniferous stock in storage, would be
applicable for container grown plants in
which nutrient levels are controlled within
close limits.
Vertical, rather than horizontal, packing of
stock has been proposed for mould control
(261. Drainage of free moisture to roots is
likely to be facilitated by a vertical arrangement, and soil deposition on foliage may be
less.
2.
Inert material, such as polyvinylchloride,
for tying plants into bundles has been advocated since mould has been observed to
commence on twine (441.
Adequate air circulation has been reported
to be important in combaning mould in
storage (38) but, although poor air circulation might permit excessive heating of material in boxes, this factor is likely to be of
greater importance with open systems of
storage.
3.
Incandescent lighting within storage chambers
has been proposed as a control measure (16,
56), but increased air circulation caused by
localized heating from the lights may have
been responsible for any favorable effect.
Experience with ultra-violet light in storage
chambers has been negative (38),
4
Packing of roots with peat moss has been
reported to control moulding (721, but the
mechanism is not clear. Possibly. in open
storage, protection from root desiccation
may have avoided predisposition to moulding.
CONCLUSIONS
1. Very little reliable information is available
from British Columbia on the identity,
damage potential and control of tree
seedling storage moulds. However, infor·
mation of varying degrees of pertinence
and reliability is available from other
regions which is of some assistance in
discerning problems and approaches of
probable value in B.C. Climatic factors
and host species grown in coastal nurseries
are expected to produce problems distinct
from those in interior locations. The
practice of storing stock for 5 to 6 months
greatly increases the damage potential
over that in regions where storage is
usually of short duration.
,
. 11 .
de,
vo·
to
ted
in
lla-
.erof
of
ers
16,
by
Ne
ct.
,ge
en
he
en
on
19.
,Ie
'Y,
ee
Jr.
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er
in
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rs
es
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,I
is
2. Detailed studies on the nature and significance of varying degrees of moulding by
specific organisms is required to distinguish accurately and assess damage.
However, operational experiences with
mortality and extensive moulding during
some years, together with the damage
studies from other areas, indicate the
advisability of regarding all moulding as
potentially serious and warranting the
«)plication of control measures.
3. Information suggests that some species
grown in B.C. can tolerate storage temperatures sufficiently low to inhibit the
growth of moulding organisms. However,
some mould may develop during the
thawing period, necessary before frozen
stock can be separated.
4. Information on low temperature tolerances of coastal species at traditional
times for commencement of lifting for
storage is required to establish the lower
limit of safe storage temperatures. Information suggests that coastal Douglas-fir
does not tolerate temperatures low enough
ta inhibit moulding and will require an
alternative approach for mould control.
5. Prospects far mould control by means of
a systemic fungicide appear good, although
special precautions to prevent the appearance of strains tolerant ta the fungicide
are advisable.
6. Cultural and sanitary approaches ta mould
control have also been suggested and
some of these appear applicable and merit
exploration.
- 12 -
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