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

FREEDOM PALESTINE FREEDOM PALESTINE FREEDOM PALESTINE
Host Protection and Defense Mechanisms 69
quiescent infections has been demonstrated in two experiments: (a)
antibodies produced against PL of C. gloeosporioides incubated with
fungal spores, inhibited anthracnose development in ripe fruits; and (b) a
nonsecreting PL mutant was found to be non-pathogenic to susceptible
avocado, indicating that even when concentrations of antifungal
compounds were reduced, PL production was required for infection
(Wattad et al., 1995).
C. PREFORMED INHIBITORY COMPOUNDS
Phenolic compounds have long been implicated in disease resistance in
many horticultural crops (Ndubizu, 1976). Some occur constitutively and
are considered to function as preformed or passive inhibitors, while
others are formed in response to the ingress of pathogens, and their
appearance is considered as part of an active defense response (Kurosaki
et al., 1986a; Nicholson and Hammerschmidt, 1992).
Activity of Preformed Compounds
Preformed resistance involves the presence in healthy tissues of
biologically active, low-molecular-weight compounds whose activity
affords protection against infection (Ingham, 1973). These are regarded
as constitutive antimicrobial barriers (Schonbeck and Schlosser, 1976;
Prusky, 1997). Phenolic compounds contribute to resistance through
their antimicrobial properties, which elicit direct effects on the pathogen,
or by affecting pathogenicity factors of the pathogen. However, they may
also enhance resistance by contributing to the healing of wounds via
lignification of cell walls around wound zones. Evidence strongly
suggests that esterification of phenols to cell-wall materials is a common
aspect of the expression of resistance (Friend, 1981).
The antifungal properties of phenolic compounds and their derivatives,
and the fact that these compounds are frequently found in the young
fruit at concentrations higher than in the ripe fruit, led to the hypothesis
that these compounds play an important role in the maintenance of
latency in the unripe fruit.
In vitro assays have shown that the phenolic compounds, chlorogenic
acid and ferulic acid directly inhibited Fusarium oxysporum and
Sclerotinia sclerotiorium, respectively. Benzoic acid derivatives have
been shown to be the best inhibitors of some of the major postharvest
pathogens, such as Alternaria spp., Botrytis cinerea, Penicillium
http://arab2000.forumpro.fr