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

FREEDOM PALESTINE FREEDOM PALESTINE FREEDOM PALESTINE
Attack Mechanisms of the Pathogen 59
maturity. In fact, the enzymatic levels recorded for the infected mutant
and hybrid fruits at their mature stage were similar to those recorded for
the infected ripening normal fruit at its mature-green stage
(Barkai-Golan and Kopeliovitch, 1989).
B. TOXIN PRODUCTION
Some pathogenic fungi secrete toxic compounds into the plant tissues,
inflicting damage on them. These compounds are frequently called
'phytotoxins'; however, this term may be misleading since it is sometimes
used to designate toxins originating in higher plants.
Toxins are pathogen-produced metabolites, most of which are
low-molecular-weight compounds that cause histological and
physiological changes in the host (Knoche and Duvick, 1987). They may
be the primary cause of the disease induced by the pathogen or they may
constitute only a part of the disease process (Scheffer, 1983; Walton and
Panaccione, 1993). Toxins that have an injurious effect only on plant
species or cultivars that serve as hosts of the toxin producing pathogen
are termed host-specific or host-selective toxins. Conversely, toxins that
affect a wider range of plant species, including both host and non-hosts of
the pathogen, are termed non-host-specific or non-host-selective toxins
(Rudolph, 1976; Mitchell, 1984). These toxins are not necessarily related
to disease initiation. Pathogen-produced toxins can also be classified,
according to Yoder (1980), as bearers of a 'pathogenicity factor', a factor
essential for a pathogen to cause disease, and those bearing a 'virulence
factor', that only enhances the extent of disease. Toxins that are
considered as virulence factors turn out to be the non-host-specific toxins,
whereas those considered as pathogenicity factors are generally
host-specific toxins (Mitchell, 1984).
Host-Specific Toxins
Host-specific toxins were for a long time considered to be the only
agents of specificity in plant/microbe interaction (Walton and
Panaccione, 1993). They represent widely differing chemical categories,
including peptides, glycosides, esters and other compounds (Macko,
1983), which function as a chemical interface between pathogens and
their host plants. However, in order to study the biosynthesis and the
role of the toxins in pathogenesis, structural information is needed. In
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