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

FREEDOM PALESTINE FREEDOM PALESTINE FREEDOM PALESTINE
222 Postharvest Diseases of Fruits and Vegetables
In accordance with the results of these steps, models to guide
management should be constructed.
Essentially, a great amount of research over the years has been
focused on developing biological control procedures based mainly on
artificially introduced selective antagonists against postharvest
pathogens. Considerable success has been achieved in this field, and a
large amount of information on the subject has been accumulated
(Wilson and Wisniewski, 1994).
A. ISOLATION AND SELECTION OF ANTAGONISTS
The production of antibiotics by the antagonist was long considered to
be the only mechanism of antagonism. In accordance with this
hypothesis, screening of antagonistic microorganisms has generally been
carried out in vitro, based on encouraging the selection of organisms that
produce growth-free antibiotic zones in Petri dishes when challenged
with the pathogen. However, such screening is likely to overlook many of
the microorganisms which may serve as potential antagonists in vivo, in
spite of failing to produce antibiotic compounds in culture (Wilson and
Wisniewski, 1989). Recent screening procedures, therefore, emphasize
the isolation of antagonists which do not necessarily produce antibiotic
compounds during their life cycle (Droby et al., 1992; Wilson et al., 1993).
Most investigators have usually preferred to study antagonistic
microorganisms which occur naturally on fruit and vegetable surfaces
and, in fact, these have proved to be a reliable source of such
microorganisms. Much attention has been given to naturally occurring
yeast species, which do not rely on the production of antibiotic
substances. They can colonize a wound for long periods, produce
extracellular polysaccharides that enhance their survival, can proliferate
rapidly by using available nutrients and are minimally affected by
pesticides (Janisiewicz, 1988).
Looking for alternative non-chemical methods to control postharvest
diseases of avocados in South Africa, Korsten et al. (1995) evaluated the
inhibitory effects of 33 bacteria isolated from avocado leaf and fruit
surfaces, against the major fungal pathogens of avocado fruits. These
pathogens included Colletotrichum gloeosporioides, Dothiorella
aromatica and species of Thyronectria, Phomopsis, Pestalotiopsis and
Fusarium. The antagonistic bacteria included various Bacillus species,
which comprise a major component of the microflora tested, and species
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