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

FREEDOM PALESTINE FREEDOM PALESTINE FREEDOM PALESTINE
Attack Mechanisms of the Pathogen 61
belong to the same group that includes the weak pathogens which attack
various harvested fruits and vegetables via wounds, following
physiological damage, or through aging tissues, without being selective
to a specific host.
Nishimura and Kohomoto (1983) related the origin of the host-specific
toxins of A, alternata to the wild strains of A. alternata. Since the
transition from a non-specific Alternaria type to a host-specific type is
associated with the production of host-specific toxins, it might be
assumed that a wild population of the non-host-specific A, alternata
initially contained toxin producers which have apparently arisen by
mutation. Mutation to toxin production probably affected an extremely
minute proportion of the field population of A, alternata before the
introduction of susceptible plants. The introduction of new susceptible
host genotypes might have led to multiplication of the toxin-producing
mutants (Nishimura and Kohomoto, 1983). Monocultures of the
susceptible plants act as a selection medium and increase the proportion
of toxic mutants. If, on the other hand, there is no chance to meet with
uniformly genetically susceptible genotypes, the property of toxin
production will not have a chance to appear and will soon be lost.
Non-Host-Specific Toxins
In contrast to the host-specific toxins, many fungi produce non-specific
toxins, which are toxic to hosts and non-hosts of the pathogen, and may
attack many different plants (Rudolph, 1976). A wide range of
non-host-selective toxins of fungal and bacterial origin have been
described and identified. Some of them seem to be necessary for
successful infection of the host, whereas others play only a secondary
role. Also, many non-selective toxins are metabolites that are toxic in
bioassays, although this property may be unrelated to disease
development. For many of the non-specific toxins, the toxicity to plants is
just another aspect of pathogenesis, acting in addition to penetration
mechanisms, enzymatic activity, recognition capability and other factors.
In spite of the damage caused by the non-specific toxins to the plant
tissues, the relationship between their synthesis and disease inducement
has not yet been proven. However, even if these toxins are not necessary
for disease initiation, they may enhance the pathogen virulence by
causing phytotoxic phenomena in the attacked tissues (Mitchell, 1984).
Rudolph (1976) lists several criteria to be investigated before deciding
upon the pathogenic role of a phytotoxic compound. Among these are:
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