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

FREEDOM PALESTINE FREEDOM PALESTINE FREEDOM PALESTINE
258 Postharvest Diseases of Fruits and Vegetables
this case, the penetration of epidermal cells by the hyphae of
Colletotrichum results in the formation of several layers of necrotic cells
around the hyphae. The necrotic cells of the host were found to be filled
with phenolic compounds which restricted fungal development to the site
of penetration and prevented further spread of the disease (Brown, G.E.,
1978).
Chitosan and Defense Reactions
Chitosan, a deacetylated derivative of chitin, has demonstrated
fungicidal activity against many fungi, including soil-borne and
postharvest pathogens. Because of its polymeric nature, chitosan can also
form gas-permeable films and create an internal modified atmosphere. It
has, therefore, the potential for use as an edible antifungal coating
material, regulating gas and moisture exchange by fruits and vegetables
(Wilson et al., 1994). For discussion of these features of chitosan, see the
chapter. Chemical Control -- Natural Chemical Compounds. However, in
addition to its direct antifungal capability and its polymeric nature,
chitosan is also a potential elicitor of phytoalexins (Kendra et al., 1989)
and of various low-molecular-weight, pathogenesis-related (PR) proteins,
such as chitinases, chitosanases and P-l,3-glucanases.
By inducing these PR proteins, chitosan application may be able to
promote defense responses in the tissues. These enzymes hydrolyze the
main components of fungal cell walls (Boiler, 1993; Bowles, 1990) and are
considered to play a major role in constitutive and inducible resistance of
plants against invading pathogens (El Ghaouth, 1994).
The inducement of such antifungal hydrolases by chitosan has been
recorded in strawberries, bell peppers and tomato fruits, in which they
have remained elevated for up to 14 days after treatment. In
chitosan-treated bell peppers the production of lytic enzymes was
followed by a substantial reduction of chitin content of the cell walls of
invading fungal hyphae, as exhibited by a reduction of chitin labeling in
the walls (El Ghaouth and Arul, 1992). Studies with bell peppers showed
that chitosan treatment caused severe cytological damage to hyphae of
invading Botrytis cinerea and interfered with the capacity of the fungus
to secrete polygalacturonases. These activities were expressed in the
reduced maceration of the host cell-wall components, pectin and
cellulose, by the pathogen (El Ghaouth et al., 1997). The induction of lytic
enzymes in harvested tissue by prestorage treatment with chitosan
could, therefore, lead to the restriction of fungal colonization. The
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