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

FREEDOM PALESTINE FREEDOM PALESTINE FREEDOM PALESTINE
240 Postharvest Diseases of Fruits and Vegetables
and coiling of T. harzianum and T, viride around the hyphae of
L. theobromae. In addition to the direct parasitism, and deformation of
some cells of the pathogenic fungus, granulation of the cytoplasm and
disintegration of the pathogen hyphal walls of L. theobromae also
occurred without intimate contact between the hyphae. It was, therefore,
suggested that the two Trichoderma species could produce antifungal
metabolites that contributed to their antagonistic activity. In fact,
culture filtrates of the antagonists were found to be more suppressive of
the conidia of the pathogen than spore suspensions, suggesting the
involvement of certain metabolites in the antagonistic effect of
Trichoderma species on L. theobromae. Both antibiosis and direct
parasitism have also been suggested to be the modes of action of
T. harzianum in reducing the incidence of postharvest stem-end rot
(Botryodiplodia theobromae), anthracnose (Colletotrichum gloeosporioides)
and brown spot (Gliocephalotrichum microchlamydosporum) on
rambutan fruits (Sivakumar et al., 2000).
An isolate of T. harzianum, which is used commercially prior to
harvest, to control B, cinerea on grapes and greenhouse crops, was found
to retard spore germination and germ-tube elongation of the pathogen.
This retardation was associated with a limiting of disease severity,
probably because of reduced penetration of B, cinerea germ tubes into the
host. However, the efficient antagonistic isolate was also found to reduce
the activity of hydrolytic enzymes of the pathogen (Kapat et al. 1998).
Levels of cutin esterase, pectin methyl esterase, exopolygalacturonase,
endopolygalacturonase and pectate lyase were all reduced when B, cinerea
was grown with the active isolate of T. harzianum, either in a liquid
culture or on the surface of bean leaves. The antagonist had no effect on
the carboxymethyl cellulase activity of the pathogen. When different
isolates of T, harzianum were used, a correlation was found between the
enzyme inhibition capacity of the antagonist and its disease suppression
efficacy, supporting the hypothesis that the inhibition of the activity of
pathogenic enzymes of JB. cinerea is a mechanism by which the biocontrol
agent affects disease (Kapat et al., 1998).
(d) Induction of the host defense mechanism. One of the
explanations offered for the mode of action of the antagonistic yeast,
P. guilliermondii lay in its ability to stimulate wound healing or to induce
other defense mechanisms in the host (Droby et al., 1992). Experiments
with Pic/iia-inoculated citrus fruit showed that the yeast stimulated
ethylene production and raised the levels of the enzyme phenylalanine
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