Post harvest diseases fruits and vegetables - Xavier University ...

FREEDOM PALESTINE FREEDOM PALESTINE FREEDOM PALESTINE
236 Postharvest Diseases of Fruits and Vegetables
nutritional conditions: it should grow more rapidly than the pathogen,
while using low concentrations of nutrients, and should be able to survive
under conditions that are unfavorable to the pathogen (Droby et al., 1991).
One can assume that the antagonist, which grows rapidly in the
wound by utilizing nutrients located there, will deprive the pathogen of
available nutrients and inhibit spore germination and germ-tube
elongation, which are the stages prior to pathogen establishment in the
tissues. Thus, it was found that the rapid development of the
antagonistic yeast, Pichia guilliermondii on grapefruit peel at a broad
range of temperatures and at various levels of relative humidity, enables
it to populate the wound within 24 h, while P. digitatum spores are still
at their initial stages of germination (Droby et al., 1992). Similarly, the
development of the yeast cells in wounds of tomato peel preceded the
development of Rhizopus stolonifer, B. cinerea or Alternaria alternata,
which had also been introduced to the same wound (Chalutz et al., 1991).
Several findings support the hypothesis that the main mechanism by
which P. guilliermondii inhibits B, cinerea and other postharvest
pathogens, is competition for nutrients at the wound site (Chalutz et al.,
1991): (1) the addition of nutrients to the wound during inoculation
markedly reduced the antagonistic effect of the yeast, and the degree of
reduction depended on the concentration of nutrients added; (2) a
marked reduction in the growth rate of the pathogen occurred only under
conditions of limited nutrition, when the antagonist cells were cultured
with the pathogen on a poor synthetic medium, and there was no growth
inhibition on a rich medium (PDA); (3) the rapid rate of growth of the
yeast at the wound site during the critical first 24 h of incubation. The
non-specific nature of the antagonistic yeast and its ability to inhibit
several wound pathogens of diverse hosts, such as citrus, apple, grape
and tomato, support the hypothesis that competition for nutrients is the
major mode of action of Pichia in decay suppression.
A similar mode of action was suggested also for non-pathogenic yeasts
of the genus Cryptococcus which have been isolated from a natural
epiphytic population on leaves and fruits of apples and pears (Roberts,
1991). These yeasts, which effectively prevented or reduced postharvest
diseases of apples, pears and cherries, were able rapidly to colonize, to
prosper and to survive in wounds for long periods under a broad range of
temperatures (0-20°C). Other features of these yeasts, which could be
important for the storage of apples, are their ability to survive in
C02-enriched controlled atmospheres and their resistance to various
fungicides, such as benomyl, sodium or^/iophenylphenate and rovral.
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