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

FREEDOM PALESTINE FREEDOM PALESTINE FREEDOM PALESTINE
Novel Approaches for Enhancing Host Resistance 257
induced accumulation of chitinase and glucanase protein (Porat et al.,
1999).
Although the potential of UV light as an inducer of defense reactions
has been proven for various fruits and vegetables, this treatment by itself
is unlikely to provide disease control comparable with that provided by
synthetic fungicides. El Ghaouth (1994) suggests that in order to extend
UV effects during a significant storage period, UV treatment should be
combined with other alternatives such as a biological control by means of
antagonistic microorganisms.
Studying the effects of UV-C illumination on the natural microbial
epiphytic population on the surface of grape berries, Nigro et al. (1998)
found a significant increase in the population of antagonistic yeasts and
bacteria on berries treated with 0.25-0.5 kJ m-2 of UV-C energy. These
results indicate that the UV-C light doses capable of inducing disease
resistance have no negative impact on the epiphytic population and may
even elevate the populations of some of the efficient antagonistic yeasts
and bacteria which may serve as natural biological control agents on the
fruit (see the chapter on Biological Control).
2. CHEMICAL ELICITORS
Among the chemical elicitors, we include those already well known, as
well as newly introduced compounds that have been found to be
associated with natural defense processes.
Ethylene as Inducer of Hypersensitive Response
Some disease-resistant plants restrict the spread of fungal, bacterial
or viral pathogens to a small area around the point of initial penetration,
where a necrotic lesion appears. This phenomenon, referred to as the
hypersensitive reaction, may lead to acquired resistance to subsequent
pathogen attack after the initial inoculation with the lesion-forming
pathogens. The ability of ethylene to induce a hypersensitive response
had already been demonstrated in the 1970s by G.E. Brown (1975, 1978):
treatment of harvested Robinson tangerines with ethylene was found to
cause a loss of chlorophyll and an increase in the content of carotenoids
in the fruit. In parallel, the treatment stimulated the formation of
infection hyphae by the appressoria of Colletotrichum gloeosporioides
present on the fruit peel, and enhanced disease development. However,
when the appressoria are removed from the peel before ethylene
application, the ethylene-treated fruit, which becomes orange in color,
develops resistance to invasion by subsequently applied fungal spores. In
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