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FREEDOM PALESTINE FREEDOM PALESTINE FREEDOM PALESTINE
Biological Control 239
Apart from the effects of direct contact of the antagonistic yeast with
the pathogen hyphae, filtrates of the antagonist cells can produce higher
levels of gluconase than filtrates of the non-antagonistic yeast, and the
gluconase activity is the cause of cell wall degradation at the sites of
attachment. These findings raised the suggestion that the firm
attachment to the fungus, in conjunction with the enhanced activity of
cell-wall degrading enzymes, may have an important role in the
biological activity of P, guilliermondii, and that its efficacy is not
dependent only on its competition with the pathogen for nutrients
(Wisniewski et al., 1991). However, there is still the possibility that the
firm attachment enhances the utilization of nutrients by the antagonist,
at the site of invasion, and thereby blocks access to the available
nutrients by the pathogen (Wisniewski et al., 1988).
Various studies with species of Trichoderma, which are known as
effective biocontrol agents of several important plant pathogenic fungi,
highlighted the role of lytic enzymes, including glucanases, chitinases
and proteinases, in the capability of the antagonist to attack the
pathogen and thus to reduce disease incidence (Chet et al., 1993;
Goldman et al., 1994, Lorito et al., 1993; Mortuza and Hag, 1999).
Trichoderma harzianum isolates were found to produce chitinases and
glucanases when grown on live mycelium of Sclerotium rolfsii and
Rhizoctonia solani in soil (Elad et al., 1982). The chitinolytic system of
T. harzianum is made up of six distinct chitinolytic enzymes: two
P-l,4-A^-acetylglucosaminidases (exoenzymes) and four endochitinases
(Haran et al., 1996). Both the levels and the expression patterns of these
enzymes are specifically affected by the pathogenic fungus attacked by
the Trichoderma. The parasitic interaction with R. solani involved the
expression of both the endochitinase activities and the exotype activity.
During the mycoparasitic interaction with S, rolfsii, however, only the
exotype activities of two P-l,iV-acetylglucosaminidases were detected. It
was suggested that the differential expression of T. harzianum chitinases
might influence the overall antagonistic ability of the fungus against a
specific pathogen.
When 15 Trichoderma isolates were tested for their antagonistic
ability against Lasiodiplodia theobromae, the greatest inhibition in dual
culture was exhibited by T. harzianum, followed by T viride (Mortuza
and Hag, 1999). For each species, however, inhibition increased with
increasing density of spores in the inoculum. All the Trichoderma
species reduced spore germination and inhibited germ-tube elongation of
L. theobromae. Microscopic investigation demonstrated direct parasitism
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