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

FREEDOM PALESTINE FREEDOM PALESTINE FREEDOM PALESTINE
Novel Approaches for Enhancing Host Resistance 261
substances, or their ability to injure the pathogen directly. In many
cases, these microorganisms, which grow rapidly and colonize wounds,
have been hypothesized to act by competing with postharvest wound
pathogens, for space and nutrients (see the chapter on Biological
Control). However, in several cases, antagonistic yeasts can also act as
inducers of resistance in the host tissue.
The antagonistic yeast, Pichia guilliermondii, which is effective in
controlling a wide variety of postharvest diseases in citrus fruits, apples
and peaches, has been shown to induce enhanced levels of phenylalanine
ammonia lyase (PAL) in citrus fruit peel (Wisniewski and Wilson, 1992),
indicating the induction of a defensive response. P. guilliermondii, as
well as an isolate of the yeast Candida famata, which is similarly
effective against Penicillium digitatum in citrus fruits (Arras, 1996),
contributes to enhanced host resistance by inducing the formation of the
phytoalexin scoparone or of scoparone plus scopoletin, respectively, in the
fruit peel (Rodov et al., 1992; Arras, 1996).
Studies with unripe avocado fruits indicated that their resistance to
anthracnose, caused by Colletotrichum gloeosporioides, was related to the
presence of an antifungal diene compound in the peel (Prusky and Keen,
1993). It has been suggested that fruit resistance to infection could be
modulated, not only by delaying the normal decline of the antifungal
diene by retarding fruit ripening, but also by increasing its synthesis.
Challenge inoculation with the avocado pathogen, C. gloeosporioides
(Prusky et al., 1990), or with a non-pathogenic mutant of Colletotrichum
magna (Prusky et al., 1994) led to increased levels of the antifungal
diene. These results suggest that the non-pathogenic Colletotrichum
mutant is capable of enhancing the natural defense mechanism of
avocado fruits, leading to the prolongation of the quiescent period of the
pathogen and thus to disease inhibition.
Wild and Wilson (1996) have recently detected a host defense reaction
in apples, which reduced decay development in fruits that had been
challenged by Penicillium expansum, the typical blue mold fungus of
apples. The application of the protein synthesis inhibitor, cycloheximide
prevented the reaction and resulted in more than 700% increase in
decay. Following cycloheximide application, inoculation of apples with
the citrus pathogen, P. digitatum resulted in green mold rot development,
although the fungus does not normally attack apple fruits. Once the
citrus Penicillium became established in the apple, it progressed at a
higher rate than that of the apple Penicillium, It was also found that if
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