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

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FREEDOM PALESTINE FREEDOM PALESTINE FREEDOM PALESTINE Biological Control 237 However, the antagonistic capacity of Cryptococcus spp. depended on their concentration as well as on the storage temperature (Roberts, 1991). Biocontrol of the gray mold on apples by two isolates of another yeast, Metschnikowia pulcherrima, was found to be reduced or totally suppressed by the addition of several nitrates. This finding supports the hypothesis that competition for nutrients plays a role in the biocontrol capability of the yeast against S. cinerea. Under culture conditions, however, both isolates of the antagonist inhibited spore germination and mycelial growth of the pathogen, regardless of restrictive nutrient conditions (Piano et al., 1997). Non-pathogenic species of Erwinia, such as Erwinia cypripedii, showed an antagonistic activity against various isolates of Erwinia carotovora, the causal agent of soft rot in many vegetables (Moline, 1991). Dipping carrot disks in a cell suspension of the antagonistic Erwinia, followed by inoculating them with active pathogenic isolates of E. carotovora, prevented the tissue maceration typically caused by the pathogen; whereas dipping the disks in a cell suspension previously heated, to inactivate the antagonistic cells, canceled the antagonistic effects. The antagonistic Erwinia also significantly reduced the incidence of soft rot caused by E. carotovora, in tomatoes and peppers, after it was introduced to a fresh wound in the fruit (Moline, 1991). Since the antagonistic strain of Erwinia did not exhibit any antibiotic activity, its inhibitory effect on disease development was hypothesized to be the result of competition for substrate in the wound site, between the pathogenic and the antagonistic Erwinia spp. This hypothesis is supported by the finding that various bacteria serve as biocontrol agents in the soil, through their ability to bind iron, thus limiting the pathogen growth in the micro-environment (Neilands, 1981; Leong, 1986). (c) Direct effect of the antagonist or its enzymes, on the pathogen. Several electron microscope observations (Wisniewski et al., 1988, 1991) showed that the antagonist yeast cells may injure the pathogen directly. Co-culturing P. guilliermondii with the pathogenic fungi, J5. cinerea or P. expansum demonstrated the ability of the yeast to attach fungal hyphae closely (Photo 7). In many instances, individual yeast cells appeared to be embedded within depressions in the hyphal wall and in some areas extensive pitting and holes were observed in the fungal hyphae (Photo 8). In contrast, co-culturing B, cinerea with a non-antagonistic yeast elicited only a loose attachment to the fungus, with no pitting in the hyphae. http://arab2000.forumpro.fr
FREEDOM PALESTINE FREEDOM PALESTINE FREEDOM PALESTINE 238 Postharvest Diseases of Fruits and Vegetables Photo 7. Coculture of Botrytis cinerea and the antagonistic yeast Pichia guilliermondii: tenacious attachment of the yeast cells to fungal hyphae. (From Wisniewski, Biles and Droby, 1991). Photo 8. Coculture of Penicillium expansum and the antagonistic yeast Pichia guilliermondii: formation of pitting and holes in the fungal hyphae. (From Wisniewski, Biles and Droby, 1991). http://arab2000.forumpro.fr
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