Non-Chemical Decrease of Some Anthracnose Effects on Mangifera

Sciknow Publications Ltd.
International Research Journal of Horticulture
©Attribution 3.0 Unported (CC BY 3.0)
IRJH 2015, 3(1):1-8
DOI: 10.12966/irjh.05.01.2015
Non-Chemical Decrease of Some Anthracnose Effects on
Mangifera indica in Tropical Highland Valleys: Implications of
Rising Sea Levels on Tropical Agriculture
Mark Anglin Harris*
College of Natural & Applied Sciences, Northern Caribbean University, Mandeville, Jamaica W. I.
*Corresponding author (Email: [email protected])
Abstract –Anthracnose transmission by Colletotrichum gloeosporioides spreads with increasing humidity. Consequently, in
some highland locations, yields of Mangifera indica are reduced by low fruit-set. But rising sea levels may push current
agriculture to higher ground. Yet, effective treatments with pesticides require almost year-long, year-round application with
potentially detrimental, unsustainable environmental effects. Though chemical treatments have increased yields, the use of
physical barriers against anthracnose transmission to initiate and increase fruit-set has not been observed in the literature. It was
hypothesized that the environmental conditions (high humidity) enhancing the proliferation of anthracnose could be avoided
with environmentally non-toxic prophylactic treatments. Mature (>20 years of age), heavily inflorescent trees which never
previously fruited at any time were chosen. Using transparent polyethylene shrouds, heavy dews were excluded from panicles,
thereby removing a major condition for anthracnose proliferation. Of 420 inflorescences so treated over a 3-year period, 108 bore
fruit which lived to harvest (i.e., 25%), compared with 0 of 1260 controls (0.00%). Nevertheless, when the polyethylene shrouds
were made airtight, all of the set fruit dropped while still immature. Premature fruit-drop is thus correlated with increasing
humidity and heat – conditions which encourage the proliferation of anthracnose. Preventing a build-up of water evaporating
from the panicles must therefore occur to prevent premature fruit-drop. This treatment promises increased M. indica yields in
tropical highland valleys, and reduced pesticides in the environment.
Keywords –Climate change, Colletotrichum-gloeosporioides , Dew, Ethylene-thiourea , Mancozeb , Panicles
1. Introduction
As an abundant source of beta carotene, vitamin A, soluble and insoluble fibre and other vitamins, Mangifera indica (mango) is
an important fruit crop in many developing countries, being well-suited to tropical lowlands, even in dry conditions (Ramcharan
& George, 1997). Hence, the mango supplies a very efficient, low-cost preventative for several eye diseases and other health
issues concerning human populations in developing countries.
As a perennial, branching, tropical evergreen, the mango tree is adversely affected by sub-tropical temperatures (Morton,
1987; Rivera-Vargas et al., 2006), and it has not thrived in moist conditions often found in tropical highland locations (Morton,
1987), due largely to fungal diseases (Pradeepa et al., 1994; Waller et al., 2007). Yet, potential effects of climate change include
the inundation of fertile tropical lowlands. Therefore, food production in the humid tropics may increasingly depend on farming
at higher altitudes.
Anthracnose, the most important mango disease (Pernezny & Marlatt, 1993) is caused by the fungus Colletotrichum
gloeosporioides, where infections starting as small brown or black lesions can coalesce to infect the whole panicle. This kills the
flowers (Pernezny & Marlatt, 1993), thereby potentially preventing the formation of fruit (Nelson, 2008) on an entire tree
(Pernezny & Marlatt, 1993). In humid highland valley locations of Jamaica above 100 metres, the ratio of non-fruiting to fruiting
trees is high >20:1, while in coastal locations it is < 5:1 (Harris, 2011, unpublished data). More than 1000 other plant species have
encountered problems with anthracnose (Moriwaki & Toyozo, 2002), especially in the tropics (Tran et al., 1998). These include
papaya (Al Eryani-Raqeeb et al., 2009), yam (Amusa et al., 2006), guava (Amusa et al., 2006) and cassava (Fokunang et al.,
2003).
Antibiotic-resistant pathogenic microorganisms have raised the demand for finding alternative strategies for reducing
environmentally unsustainable chemical pesticide applications (Obeidat et al., 2012). Though target-specific chemical
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International Research Journal of Horticulture (2015) 1-8
fungicides including carbamates such as Mancozeb have proven effective against C. gloeosporioides, their health risks as
cholinesterase inhibitors (Wesseling et al., 1997; Manahan, 2008) discourage their use. Mancozeb degrades rapidly in soils
(Hanumatharaju, 2004), and deep ground water may remain unaffected (Geissen et al., 2010). However, surface and sub-surface
water in a Costa Rican location under banana crops have been highly polluted with ethylene-thiourea (ETU), the main metabolite
of mancozeb (Geissen et al., 2010), and a thyroid carcinogen and potent teratogen in rats (NTP, 1992). The potential for
environmental harm is significant because successful pesticide applications for anthracnose affecting M. indica are effective only
after frequent applications over several months (Huang et al., 2012). Further, Shukla and Arora (2001) showed that mancozeb
and its metabolites can (1) cross the placental barrier (2) produce DNA damage (3) initiate tumors in fetal cells.
Less harmful alternative strategies have been suggested. Defoliation before harvesting reduced weevil infestation and fungal
decay of fruit (Dukuh, 2011). However, for M. indica the anthracnose problem occurs before fruit-set. Defoliating before
flowering in such a case would be beneficial but for the fact that leaves are required precisely at that stage to manufacture the
energy required for flowering. Coldness (Waller et al., 2007), heavy dews (Waller, 1992) and raindrop impact (Harris, 2011,
unpublished data) singly or in combination have been suggested reasons for lack of fruit-set. Anthracnose-induced
non-fruit-bearing of M. indica trees can be observed at altitudes as low as 30m above MSL (Harris 2011, unpublished data)
The risk to aquatic ecosystems, surface water sources, and human health (Khera, 1973; Krieger, 2001) therefore encourages
alternatives to heavy carbamate applications. Current knowledge shows that anthracnose spreads with increasing humidity.
Though chemical treatments have been successfully used, the use of successful physical barrier devices against anthracnose
transmission has not been cited in the literature. Therefore, effects of altitude, coldness, heavy dews, and raindrop impact, on
fruit-set of M. indica protected by a physical barrier will be investigated in this study.
1.1. Aim of study
To reduce pesticide applications, the aim of this work was to determine the conditions causing low fruit-set and to alleviate those
conditions with a non-toxic treatment. The effectiveness of complete polyurethane tree wraps to protect trees from frost effects
has been confirmed (Ferrer, 2014). Such wraps could also exclude heavy dews on inflorescent M. indica trees from either
nocturnal radiation or advection. However, such a tree-wrap device would lock in conidia already in the canopy, and the dispersal
of the conidia within canopies between branches of the same tree, especially in highly humid conditions, could infect and rapidly
destroy inflorescences (Waller, 1992). Therefore, by preventing the escape of such spores by air currents, a tree-wrap intended to
exclude dew could actually exacerbate the branch-to-branch pathogen transmission process. The suggested alternative is to
surround individual panicles, thereby preventing cross-infection from the same tree.
1.2. Hypothesis
M. indica fruits well in all local coastal locations regardless of rainfall regime. But nocturnally cool, moist conditions in the
highland valleys of Jamaica are conducive to proliferation of anthracnose. In the daytime, coastal locations facing the North East
Trade Winds are often cooler than many highland interior locations. On the other hand, mango trees rarely fruit at interior valleys
where rainfall is low - yet fruit prolifically in coastal zones, even in the north-east where rainfall is heavy. It is therefore
hypothesized that neither heavy rain nor low daytime temperatures per se causes low fruiting of M. indica trees in interior
highland valleys, but low nocturnal temperatures producing high relative humidity are the major factor.
2. Materials and Methods
2.1. Date and location of research
The duration of this research was 20 Jan, 2009 – 20 Sept, 2012, at two inland valley regions in Jamaica (Fig.1). Within the
northern section of the Blue Mountains, the Upper Rio Grande Watershed (URGW) is located in north eastern Jamaica, between
1815' and 1800' north latitude and 7615' and 7645' longitude (Fig.1).
The region spans about 30,970 hectares at altitudes above 100m and is dominated by the North-east Trade Wind zone (Fig.1).
High rainfall exceeding 2000 mm (Jury et al., 2007) and mountainous topography have produced spectacular landscapes
resulting from the interplay of hills and valleys and many rivers, the largest of which is the Rio Grande. The major rainy season
from October until mid-January often coincides with the M. indica inflorescence development period, and world records for
intensity of rainfall have been recorded at Mill Bank, within the eastern study area in the Blue Mountain Range (Jury et al., 2007).
Despite acquisition of abundant inflorescences and luxuriant vegetative growth, fruit yields for M. indica here are usually very
low or non-existent in the valleys. Contrastingly, on the hill-slopes at higher elevations, yields are substantially higher, and
everywhere in the region, substantially improved mango fruit-set occurs in the El Nino dry years (Harris, unpublished data,
2011).
International Research Journal of Horticulture (2015) 1-8
3
Figure 1. Two study areas [low fruiting] producing nocturnal dew in the frequency ratio of 8:1 as compared to coastal locations
[high fruiting].
2.2. Study area: Manchester Plateau, south-western Jamaica
Like the Upper Rio Grande Valley, the Mandeville Region is surrounded by a ring of hills. It is within an area consisting of a
deeply dissected tertiary limestone plateau containing pockets of deep, ferallitic red soil exhibiting a low cation exchange
capacity, with adjacent thinly soiled slopes (Harris & Omoregie, 2008). A small seasonal temperature range of < 10 ºC (25 – 34
ºC) prevails. Diurnal temperature differences however, can be as high as 17 ºC. Two rainy seasons, in May-June, and
October-November, along with tropical summer depressions yield 50 – 70 inches (275 cm) of rain per year (Jury et al., 2007).
However, heavy rains in January-March (the mango flowering season) are rare (Jury et al., 2007). Though both study areas are
comparable in altitude (Fig.1), night-time humidity, and (low) fruit yield, the Upper Rio Grande Valley (URGV) exhibits
substantially greater rainfall intensity and more than double the annual total. The cooler of the two study areas (Manchester
Plateau) is the less rainy, but both areas are nocturnally cooler than all local coastal districts.
2.3. Dew-excluding polyethylene shrouds
As panicle inflorescences were to be the subjects in the population of this study, a sample size comprising blossom panicles
(which contain the inflorescences), and not one of individual trees was to be chosen. As this study was therefore to be a
comparison among panicles and not among trees, a large number of panicles (and not a large number of trees) was required.
Panicles and not trees per se were to be studied.
For the consecutive years of 2009, 2010, and 2011, 0.5mm thick polyethylene shields were fabricated and each fitted over a
total of 35 blossoming panicles on >20-year-old non-bearing mango trees. These trees had never fruited at any time in their >20
years of existence. Polyethylene was chosen for its excellent chemical resistance. It should retain its integrity during exposure to
plant exudates for the duration of this study and aromatic hydrocarbons dissolve it only at elevated temperatures. Non-fruiting
status of the trees was based on eyewitness accounts.
Figure 2. A polyethylene shroud used to exclude dew from inflorescences.
When this tree [right] prolifically flowered, polythene shrouds were fitted. Yet, six weeks later, no fruit was observed on the tree except within the two
non-air-tight dew & rain-excluding polyethylene shrouds [right]. All seven fruits seen here lived until maturity at harvest.
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International Research Journal of Horticulture (2015) 1-8
Each polyethylene shroud was affixed during the week of flower initiation with duct-tape over a 30 cm diameter hollow
½-sphere frame made from 2 mm gauge ―clothesline‖ galvanized iron wire (Fig.2). A polyethylene shroud was used to exclude
dew from inflorescences. In still conditions, dew occurs predominantly in a downward direction, hence dew forms on the shrouds
instead of on the inflorescences.
The wire frame support ensured structural integrity against the weather and transparent polyethylene used because M. indica
requires adequate light for photosynthesis (Morton, 1987). The completed shroud (PC1) was anchored with wire struts over a
section of a nearby branch of the tree, or on the same branch bearing the treated inflorescence. Another set of 35 plastic shields
(PC2) were installed but these were airtight, thereby locking in moisture from transpiration and leaf guttation. Plastic shrouds (5)
with no sides but a flat top 10 cm above the panicle (PC3) excluded raindrop impact but not dew, and lastly, those with no poly
ethylene at all were exposed totally to the weather (PC4). There were thus four treatments per year, and each treatment consisted
of 35 replicates for a total of 140 samples. The experiment was repeated each year for three consecutive years.
2.4. Mass of dew measured
To measure mass of dew, water which formed on leaves was removed using pre-weighed 15 x 15 cm absorbent cotton cloths to
wipe off the dew in the early mornings at the same time each day (mornings). The cloths were immediately placed in individually
pre-weighed plastic zip-lock ® bags and the weight of water calculated by difference.
2.5. Statistical analysis
Using a table of random numbers, an equal number of control panicles were randomly selected from each tree in each year.
According to the null hypothesis, there were no differences among the means of the outcomes of treated panicles. The objective
was to find treatment means that were significantly different from one another. Only one panicle treatment resulted in fruit-set to
any degree throughout the three years. This rendered the results of all the unsuccessful treatments effectively identical. Welch’s
t-test comparing means of two populations of unequal variance was used. Student’s t-test comparing means of two populations of
comparable variance was adopted for the comparison of dew collected from the upper and lower surfaces of leaves at the (P < .05)
level of significance. For assessing the variation in the number of days before onset of observable anthracnose infection for four
panicle treatments, Tukey’s HSD was used.
3. Results and Discussion
Panicles retaining fruit
3.1. Fruit-set
Prior to this project, no flower on any treated tree had ever produced fruit. Fruiting by M. indica is rare not on highland slopes,
but in highland valleys of Jamaica. However in the three years of this experiment in both study areas, 98 panicles which excluded
dew but were not air-tight (PC1) carried fruit (>20%) to harvest (Fig. 3). Only one other treatment, the PC2 (dew & no rain +
air-tight) carried fruits, 69 fruits to 4 weeks (16.5%; Fig. 4). This compared with 0 (0.00%) of remaining controls (Fig.4).
60
50
40
30
20
10
0
2009
2010
2011
1
2
3
4
Weeks after fruit-set [x 4]
Figure 3. Number of panicles [of 140] which retained fruit after each 4-week period until harvest
Only the successful treatment = no dew no rain non-air-tight shrouds [PC1] is depicted on chart
Fruits which carried to maturity were located exclusively under the dew-excluded non-airtight poly-ethylene shrouds (PC1)
(Fig.4).
No other treatment bore fruit at any stage, and similar results were observed for each of the three years of the experimental
International Research Journal of Horticulture (2015) 1-8
5
period (Fig.4).
40
35
30
25
20
2009
15
10
2010
5
2011
0
1
2
3
4
Treatments: 1 = no dew no rain [study area #1] ; 2 = dew + rain; 3 =
dew, no rain [air-tight], 4 = no dew no rain [study area #2]
Figure 4. Number of panicles [from 140 per treatment] which still retained fruit at 4 weeks
Inflorescences which developed a darkened colour perished before fruit-set occurred
The PC1 treatment did not retain all fruit until harvest; about 25% of the fruit dropped before maturity (Fig.3). Nevertheless,
the treatment increased fruitage (to maturity) on inflorescences from 0% to >20% per year for each of three consecutive years.
Moreover, each successful panicle shield produced between 3 and 20 fruits. These results are from mature trees which had never
set fruit in any year preceding this experiment.
A proportion of fruit exhibited a blackened coloration several days after fruit first appeared. This occurred not only twice as
quickly on the PC2 samples as on the PC1 samples, but was apparently responsible for the premature fruit drop from all panicles
treated by PC2 (Fig.4). Thus in each of three years the PC1 panicles outperformed all other treatments. Only where dew and
raindrop impact were concurrently excluded by non-air-tight polythene caps did fruit last to maturity in any year (Fig.3).
Air-tight exclusion of both rain and dew (PC2) produced some fruit, but all died before achieving maturity (Fig.3). Avoiding
raindrop impact only (PC3) or concurrently admitting raindrop impact and dew (PC4; i.e., total exposure to the weather)
produced no fruit at any time (Fig.4).
On close inspection of the air-tight samples, water droplets were seen on the inside of the polyethylene shields after only one
week of treatment, and by week 3 some leaves inside appeared dry and brown. They were apparently desiccated by the high
temperatures attained by trapped infra-red rays. This contrasted with the perpetually green leaves of all other treatments.
In this study, it is the premature destruction of flowers that prevent fruit-set. Therefore the most singular objective here was to
prolong the viability of inflorescences. The PC1 and, to a lesser extent PC2 (airtight), retained the integrity of blossoms for an
average of 40 days and 17 days respectively for each year over a 3-year period, compared to just 7 days for all other treatments
(Table 1).
Table 1. Days elapsed before observable anthracnose infection [brown or blackish coloration] on inflorescences
Year
2009
2010
2011
Mean
SEM
Dew, rain impact excluded;
not air tight [PC1]
44c
42c
46c
44c
7.9
Rain excluded,
air-tight:[PC2]
18b
17b
16b
16b
0.9
Rain excluded,
dew included [PC3]
6a
8a
7a
6a
Total exposure
to rain & dew:[PC4]
8a
7a
6a
5a
Values with different letters are significantly different for Tukey’s HSD (P< 0.05)
Without protection from direct raindrop impact, the dew-included-rain-excluded treatment (PC3) did not extend the life of
blossoms at all and in that respect was similar to the totally exposed (PC4) treatment. Therefore a factor(s) other than physical
contact with raindrops destroyed the blossoms. As exclusion of raindrops with dew inclusion did not increase inflorescence
(blossom) longevity, it must be concluded that dew in sufficient quantities decreased the lifespan of blossoms. On the other hand,
by substantially increasing the lifespan of blossoms, dew exclusion (PC1) provided conditions necessary for a greater chance of
fruit-set.
Compared with the inland highland valley stations, total non-dew days were far greater at the coastal locations in the ratio of
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International Research Journal of Horticulture (2015) 1-8
at least 7:1 (Table 2).
Table 2. Total Days of Dew, & Rain at Inland* (>200m high), & Coast, at Blossoming Time (December - March) for M. Indica
Location
Mill Bank*
Moore Town*
Mandeville*
Mean
Port Antonio
Kingston
Morant Bay
Mean
Rain
32c
26c
6a
[ 22c]
10b
5a
5a
[7.9a]
Dew only
55d
58d
76e
[62.6d]
15b
4a
3a
[7.3a]
No dew
3a
4a
6a
[4.6a]
63d
79e
80e
[75e]
Values with different letters are significantly different (P< 0.05)
Leaf growth of M. indica is in an approximately horizontal position. On weighing dew which gathered on the upper or lower
surfaces of leaves (of M. indica and several species with similarly positioned leaves) each morning, it was found that each upper
leaf surface contained at least 6-fold the mass of water found on the lower surface (Table 3). Moreover, in several cases no
measurable dew-water was found on lower surfaces of exposed horizontally positioned leaves (Table 3). Similarly, dew was
observed on the outer surfaces of all the polyethylene shields but very little dew on the inner surfaces.
Table 3. Ratio of Dew Mass on Upper Surface/Lower Surfaces of Leaves at 6 a.m.
Location
Various species including
those of M. indica [average values]
Upper Rio Grande Valley
6:1
Mandeville Plateaux
8:1
Mean
7:1
Comparing two treatments (PC1 and PC2), which to varying degrees excluded both moisture and fungal spores, more water
vapour and heat was trapped in the air-tight (PC2) panicle shields. The trapped condensed water in the air-tight shields of PC2,
plus the extra heat accumulated therein during the days most probably accounts for the death before reaching maturity of the PC2
fruit. Dickman et al. (1982) found that C. gloeosporioides requires free water for its spores to germinate on the plant, and that
among the favourable environmental conditions are high temperatures and high humidity. But sunlight and low humidity rapidly
inactivate spores, as observed by Gould & Peterson (1994), who report negligible germination below 97% relative humidity, but
release of spores from acervuli occur when there is an abundance of moisture (Sutton, 1980). Thus almost 100% humidity is
necessary for germination of such spores. Further, on old lesions kept in prolonged wetness the fungus produces conidia, even
without acervuli (Denham & Waller, 2008; Morya, 2009). In addition, they observed that
1. the higher the leaf wetness duration and the inoculum concentration are, the greater are fungal sporulation and damage;
2. although heavy rains and high winds favour conidial dispersal, such scattering can also occur in drizzle, and
3. heavy dew is sufficient to ensure downward dispersal of the conidia.
In this study, the ratio of dew mass on the upper surfaces of exposed leaves to that on the lower leaf surfaces at all times
exceeded 5:1 (Table 3). This showed that the heavy dews in both highland locations coalesced in a predominantly downward
direction, and north-eastern Jamaica (the location of the URGW) exhibits the highest frequency of early morning drizzle in
Jamaica (Harris, 2010). The dome-shaped design of the dew-reducing plastic shields covering the panicles of this study therefore
prevented not only downward dew, but downward fungal contamination of the inflorescences. Further, as stated earlier, diurnal
temperature ranges are higher (hence dew-producing) than seasonal ranges in both study areas. This accounts for the great daily
disparity (>4:1 ratio) in the number of dew producing mornings between inland highland stations (large diurnal temperature
range) of Jamaica and the more seasonally equable coastal stations (small diurnal temperature range). In this regard, Bailey (2006)
states that M. indica blossoms best in ―dry winters‖ (i.e., not necessarily ―hot‖ winters). All the above observations probably
explain why old lesions of C. gloeosporioides were observed on mango leaves of abundantly fruiting trees in Kingston, a hot, dry
coastal district, just as they were on non-fruiting trees in the nocturnally cooler highland valley districts of Jamaica.
3.2. Slope and insolation
In sections of the Upper Rio Grande Valley (URGW), Harris (unpublished data, 2011) repeatedly estimated by eye observation a
ratio of 1:3 for fruiting to non-fruiting mango trees on hill slopes, but only 1:15 for valley floors. But dew varies directly with
altitude, and Waller (1992) found that severe anthracnose infection is particularly observed at high altitude for coffee cultivation
in the tropics. The increase of M. indica fruit-set on the slopes of URGW was therefore against expectations, considering that
orographic rainfall (particularly in north-eastern Jamaica) increases with altitude, and because Agam & Berliner (2006) report
greater night-time radiation cooling on slopes compared to lower levels. They also observe that the primary condition for the
International Research Journal of Horticulture (2015) 1-8
7
formation of dew is a lower or equal temperature of the surface (on which condensation takes place) compared to that of
dew-point. Therefore colder, heavier air rolling off slopes reduces ground temperature in adjacent valley floors as heat is
conducted away from that ground, up through the colder air contacting it. Such heat diffusion thereby produces dew on highland
valley floors. Rapid radiative loss of such heat from the air perpetuates the conduction.
3.3. Drifting spores or raindrop impact?
Bailey (2006) noted that healthy M. indica may typically drop up to 40% of immature fruit as a natural response to heavy fruiting
to conserve nutrients due to the tree’s control mechanisms which bring only a particular quantity of fruit to maturity. However,
Jeffries et al. (1990) found that ―later infections‖ produce lesions on young fruit that commonly cause fruit shedding. It is
difficult to ascertain which of these explanations is correct for any particular fruit-shedding tree. Nevertheless, epidemiological
research on C. gloeosporioides in the tropics indicate that anthracnose epidemics on perennial crops are due to disease
transmission from various tissues of the same tree canopy (Waller, 1992). For example, splashing from rain is a common means
of spreading fungal spores of C. gloeosporioides (Dickman et al., 1982), and disconnection with the ground and with other plants
increases suppression of fungi transference in green houses (Kelaniyangoda et al., 2011). Thus, as each treated tree in this study
had been sufficiently distanced from other trees to avoid cross-infection, the above combined evidence indicates that fungal
spores transported within the same tree made contact with unprotected blossom panicles.
Having settled on the panicles for wet periods often exceeding 12 hours (not necessarily including rain per se), a condition
which favours infection (OISAT,2013), spores of C. and not raindrop impact, destroyed the inflorescences.
3.4. Implications for pesticide applications
Using pre-inflorescence bi-weekly fungicidal treatments until several weeks after fruit-set, Bailey (2006) effectively prevented
anthracnose-induced fruit-drop. Yet, as already stated, the potentially harmful effects of dithiocarbamate fungicides to
near-surface and surface water supplies should require careful consideration. It is suggested that the technique of this study
promises increased yields and reduced fungicide application rates in the environment. Moreover, Dal Pogetto et al., (2012)
showed that often, single pesticide applications can provide the same reductions as multiple ones. Furthermore, as
humidity-induced anthracnose affects several other tropical crops (Phuong et al., 2009), reductions in the use of chemical
treatments based on the findings of this study are even more promising.
4. Conclusion
At higher elevations in tropical developing societies, insufficient fruit-set reduces yields of M. indica. Yet, risks of rising sea
levels destroying large tracts of low-altitude land agricultural activities may push agriculture to higher ground. The exclusion of
heavy dews using non-toxic transparent polyethylene shrouds produced fruit on previously non-fruiting trees. This treatment
succeeded by avoiding the condensation of dew on inflorescences.
Results of this study prove that fruit-set failure of mango blossoms is not caused by raindrop impact. Conversely, by
minimizing the moisture needed for assimilation of nutrients by fungi, dew reduction produced fruit for 30% of the panicles,
thereby significantly increasing yields. A combined regime of protective shields and a lower application rate of pesticides
promise further increases in crop yield, while reducing deleterious effects on water resources and fauna. This treatment also
promises higher yields for other tropical tree-crop species including coffee, papaya, yam and guava.
Acknowledgements
The author wishes to thank the Northern Caribbean University (Research Fund) for facilitating this project. The following
persons are gratefully cited for the use of their mango trees for this project: Mr. Lemuel Brady from the Department of Biology
and Chemistry (NCU), Mr. Gad Onywere (graduate student, NCU), Mr. Palmer (Mandeville).
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