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

FREEDOM PALESTINE FREEDOM PALESTINE FREEDOM PALESTINE
Chemical Control 183
may form a protective barrier on the surface of the fruit and reduce the
supply of oxygen for enzymatic oxidation of phenohcs (Zhang and
Quantick, 1997). Apphcation of a chitosan coating also delayed, to some
extent, the increase in decay of stored litchi.
Chitosan application was also found to stimulate the formation of
structural defense barriers in bell peppers and tomatoes. These included
the induction of callose synthesis, thickening of host cell walls, formation
of papillae and plugging of some intercellular spaces with fibrillar
material, probably impregnated with antifungal phenolic-like compounds
(Wilson et al., 1994).
However, chitosan also exhibits a direct fungicidal activity and can
affect postharvest pathogens by suppressing their growth: it inhibited
spore germination, germ-tube elongation and radial growth of B, cinerea
and K stolonifer, inducing cellular alterations and damage in culture (El
Ghaouth et al., 1992). Similarly, exposure of Sclerotinia sclerotiorum to
chitosan resulted in hyphal deformation and restricted mycelium growth
(Cheah et al., 1997). Trials with chitosan-treated bell peppers indicated
the damage caused to B. cinerea hyphae, exhibited as changes in the cell
wall and by cytoplasm decomposition (El Ghaouth et al., 1994).
Examination of sections from chitosan-treated tissue of bell pepper fruits
by transmission electron microscope revealed that chitosan prevented the
disintegration of host cell walls by the pectolytic enzymes of B. cinerea
(El Ghaouth et al., 1997). This was indicated by the preservation of pectic
binding sites and the regular intense cellulose labeling over host cell
walls pressed against fungal cells. In addition to reducing the production
of polygalacturonase by the pathogen, chitosan also caused severe
cytological damage to invading hyphae. These phenomena may explain,
at least in part, the limited ability of Botrytis to colonize the fruit tissues
in the presence of chitosan (El Ghaouth et al., 1997). In parallel, chitosan
may also induce the formation of antifungal hydrolases, such as chitinase
and P-l,3-gluconase, which may lead to the reduction in the chitin
content of fungal cell walls and the stimulation of various structural
defense barriers in fruits such as bell peppers and tomatoes. Because of
these characteristics, chitosan has been hypothesized to be an elicitor of
resistance in harvested crops (see the chapter on Novel Approaches for
Enhancing Host Resistance - Induced Resistance).
Gel derived from Aloe vera plants has been found to have antifungal
activity against four common postharvest pathogens: Penicillium
digitatum, P. expansum, B. cinerea and A. alternata. The natural gel
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