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The following points highlight the seven major factors affecting disease development. The factors are: 1. Inoculum Level 2. Cultivars 3. Cultural Practices 4. Crop Rotation 5. Environmental Conditions 6. Fruit Ripening Stage 7. Harvesting.
Disease Development: Factor # 1. Inoculum Level:
An assembly of vital pathogen and its suitable host in favourable environment should take place for a successful infection to occur. However, successful infection may be dependent on the level of the inoculum (generally comprising fungal spores or bacterial cells) available.
Gaumann (1946) have claimed that a minimum number of pathogen spores are necessary to establish certain diseases even under favourable conditions.
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Such a theory denies, for many fungi, the possibility of a single spore infection and has not been accepted by other investigators. For Post-harvest pathogens, which depend on a wound to enable them to penetrate into the host, it has generally been accepted that disease development is related to both the pathogen spore load on the fruit or vegetable surface and the availability of wounds for penetration.
Regular sanitation in the field, packinghouse and in storerooms will all contribute to the reduction of the spore load on the harvested produce, while careful handling and prevention of mechanical damage helps to reduce the number of entry points for the pathogen.
However, the amount of inoculum present is closely related to weather conditions during the growing season, particularly when the spores are dispersed by rain as in the cases of Gloeosporium and Phytophthora species. An interaction between wounding and inoculum level has been described for the brown rot fungus Monilinia fructicola on stone fruits.
The presence of a high level of fungal inoculum on the fruit surface and the penetration of the unwounded fruit can take place through the stomata or directly through the peel.
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The inoculum level of the pathogen may determine the success of biological control of Post-harvest diseases with antagonistic microorganisms. The efficacy of the antagonistic microorganism in reducing decay has frequently been affected by the inoculum levels of both the pathogen and the antagonistic microorganism.
Disease Development: Factor # 2. Cultivars:
The initial preharvest factor which may affect disease development is cultivars which are vary greatly in their susceptibility to diseases. Differences in cultivar characteristics can markedly affect the keeping quality of fresh produce. Melons with a thick skin and firm texture are better than others to withstand in rigors of harvesting and handling.
Variability in Post-harvest decay among apple cultivars has been related to differences in wounding resistance of their skins, a feature which may be of major importance for decaying pathogens that depend on a wound to initiate infection.
Disease Development: Factor # 3. Cultural Practices:
They can reduce the inoculum level through sanitation or to produce conditions less favourable for disease development by modifying the canopy microclimate. Practices such as pruning of fruit trees and destruction of crop debris markedly affect the survival of pathogenic microorganisms. Application of preharvest fungicides can directly reduce the level of infestations.
However, preharvest chemical sprays with the same chemical that is designated for Post-harvest application, can enhance the production of new fungal strains resistant to that fungicide. Plant spacing within the row may also affect the incidence of rots.
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Legard (2000) found that wider spacing reduced Botrytis rot in strawberries compared with narrower spacing. This may be due to the increased number of target hosts available in closer spacing intercept more inoculum or many fruit may escape timely harvesting and thus contribute to increased levels of inoculum.
Disease Development: Factor # 4. Crop Rotation:
Rotations can reduce the source of infection and thus influence the quality of the harvested commodity by affecting the health of the subsequent crop. Field nutrition can have an impact on the development of storage decay.
Thus, the rapid development of bacterial soft rot in tomato fruits depends on the application of nitric fertilizer in the field to a great extent and the resistance of pears to Post-harvest decay increases after the nitrogen and calcium nutrition.
Disease Development: Factor # 5. Environmental Conditions:
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These may affect the pathogens directly. Many pathogens persist in soil or survive on plant debris in the field, from which winds and rain may be directly responsible for their dispersal to potential hosts.
Pathogens, such as Phytophthora spp. which infect potato tubers or citrus fruits are actually dependent on rainwater for spores germination and initiation of infections. In fact, the percentage of brown rot caused by Phytophthora parasitica in orange orchards was directly related to the amount of rainfall during infection period.
Disease Development: Factor # 6. Fruit Ripening Stage:
The susceptibility of harvested produce to pathogens primarily depends on ripening stage at the time of picking. Several types of fruits are more susceptible to injuries and as they ripen, become more susceptible to pathogen attack.
Various tissue characteristics such as the acidity level, turgidity of tissues, nutrient availability and change in senescence at ripening stages separately or in combination enhance the susceptibility to diseases.
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Other factors affecting the impact of the ripening stage on disease susceptibility involve the enhanced virulence of the pathogen, weakened host resistance and protection. One of the primary factors enhancing the susceptibility of the fruits to infection is the enhanced susceptibility of the plant cell walls to the activity of enzymes produced and secreted by the pathogens.
A strong link between the ripening stage of a fruit and its sensitivity to decay may explain as chemical substances stimulate ripening and generally enhances the decay. A classic example of conditions that stimulate ripening is the exposure of various citrus fruit cultivars to low concentrations (50 ppm) of ethylene for degreening.
Ethylene treatments are applied commercially at the beginning of citrus fruit picking season to degreen the fruits that have reached maturity but have not yet developed the desired colour.
However, this economically important procedure which enhances chlorophyll decomposition and exposes the yellow, orange or red colour in the fruit peel is accompanied by enhanced sensitivity of the fruits to decay.
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It was found that along with the enhancement of ripening, ethylene stimulates senescence and disruption of the stem end button, consequently activating the quiescent infection of Diplodia natalensis at this location and leading to increased incidence of stem-end rot.
Disease Development: Factor # 7. Harvesting:
Harvesting by hand is the predominant method for fruits and vegetables intended for fresh produce market. Proper training of pickers for selecting optimal maturity stage of commodity can keep damage to a minimum extent. Mechanized harvesting, even used correctly can cause substantial damage to the commodity, which may serve as suitable areas of penetration for wound pathogens.
It is, therefore, confined mainly to less vulnerable commodities such as carrots, potato or to crops which are proposed for immediate processing. The time of harvesting during the day may also affect the keeping quality of the produce. For most crops the cool hours of the night or the early morning can be advantageous.
The harvesting date may be of great significance for fruits proposed for prolonged storage. However, despite the use of various criteria determining the appropriate stage of maturity such as colour, size, shape, flesh firmness, content of starch, sugar and juice for the prediction of optimal harvest date is often imprecise.
