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

FREEDOM PALESTINE FREEDOM PALESTINE FREEDOM PALESTINE
60 Postharvest Diseases of Fruits and Vegetables
other words: the structure of a toxin yields clues about the biosynthetic
process and the mechanism of action.
Among the microorganisms producing host-specific toxins, we find
pathogens that survive saprophytically when they lose their capacity to
produce the toxin (Macko 1983; Scheffer 1983). These include various
species of Helminthosporium and Alternaria, as well as other fungal
species not related to postharvest diseases. Among the Helminthosporium
species we find: H, sacchari, the causal fungus of eyespot disease of
sugarcane, which is the producer of HS-toxins; H. victoriae, which
attacks oats and produces HV-toxin; H, maydis, the causal fungus of
specific blighting on leaves of sensitive corn, which is the producer of
T-toxin; and H. carbonum, which attacks corn and produces HC-toxin.
The corn fungus Phyllosticta maydis produces PM-toxin, a host-specific
toxin similar to but not identical to T-toxin. The pathogenic species of
Alternaria in this class were identified mainly on the basis of their
specific host. They include: Alternaria alternata f. sp. kikuchiana
(Alternaria kikuchiana), the causal fungus of black spot disease of the
Japanese pear, which produces AK-toxin; Alternaria alternata f. sp.
lycopersici, the causal fungus of stem canker disease of the tomato, which
produces AL-toxin; A. alternata f. sp. mali (Alternaria mali), which
attacks leaves, shoots and fruits of apples and produces AM-toxin; A.
alternata f. sp. citri (Alternaria citri), which attacks rough lemon fruits
and tangerines and produces AC-toxins; and A, alternata f. sp. fragariae
(Alternaria fragariae), which attacks strawberries and produces
AF-toxin. A toxin released from germinating spores of Alternaria
tenuissima, causing leaf spot of pigeon pea in the field, has also been
found to have host-specific properties (Nutsugah et al., 1994). Each of
these fungi has its specific toxin that may be responsible for the selective
pathogenicity of the fungus.
According to Nishimura and Kohomoto (1983), the host range of the
pathogenic strains of Alternaria is limited to cultivars that are
susceptible to their host-specific toxins, while other cultivars are highly
resistant. However, as reviewed by Knoche and Duvick (1987), such a
definition might be too sharp, since AM-, AK-, and AF-toxins were able to
exert a slight effect on hosts other than their specific hosts. Most of the
Alternaria species or subspecies are similar morphologically and are
classified today in the extended group of Alternaria alternata, being
considered as pathotypes of this fungal species. Although these fungi are
generally not included among the common postharvest pathogens, they
http://arab2000.forumpro.fr