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

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FREEDOM PALESTINE FREEDOM PALESTINE FREEDOM PALESTINE Biological Control 223 of Corynebacterium, Erwinia, Pseudomonas, Staphylococcus, Vibrio and Xanthomonas. Of these, B, subtilis was the most effective antagonist. The search for antagonistic fungi against Colletotrichum musae, the causal agent of anthracnose of banana, revealed that almost all the fungi isolated from banana fruit surfaces had at least some antagonistic effect against the pathogen. The greatest antagonistic activity was recorded for Alternaria tenuissima and Acremonium strictum, that effectively inhibited Colletotrichum growth, both in vitro and on banana fruits (Ragazzi and Turco, 1997). Searching for bacteria antagonistic to Botrytis cinerea, the main pathogen of strawberry fruits, Moline et al. (1999) showed that most of the 52 bacterial isolates that were obtained from the surface of 'organically grown' strawberry fruits had some ability to inhibit the growth of the fungus. Eleven of these, which were found capable of interfering with conidial germination or fungal growth in vitro, were selected for additional screening tests on JB. cmerea-inoculated strawberry fruit. These isolates were identified as strains of Pseudomonas putida and Chryseobacterium indologenes. Three of the best isolates (two Pseudomonas and one Chryseobacterium) were also found to reduce the incidence of gray mold rot on fruits under field conditions (Moline et al., 1999). Wilson et al. (1993) developed a method for the isolation and screening of yeast antagonists. The method is based on the application of washings to fruit wounds, which are then challenged with the fungal spores; following a few days of incubation, microorganisms are isolated from the non-infected wounds. Under these conditions, the predominant microorganisms on the culture medium were of yeast species. Tests for antagonism were performed with pure cultures under in vivo conditions. Lorito et al. (1993) attempted to isolate selective biocontrol agents by utilizing media with purified polymers, such as chitin, in order to detect chitinase activity exhibited by the antagonistic microorganisms. Isolation based on chitinase activity of the antagonist offers some level of selection for properties potentially important to fungal control, but effective microorganisms which do not produce chitinases could be missed by the screening process. Some level of selection could also be achieved by using media made of pasteurized or autoclaved homogenates of the pathogens (Valois et al., 1996). This method may select only the microorganisms that can live on specific cellular content. A simple selection method for isolating bacterial antagonists to B, cinerea, which is both a pre- and a postharvest pathogen of strawberries, was recently developed by Moline et al. (1999); it is based on the ability of selected antagonistic bacteria to http://arab2000.forumpro.fr
FREEDOM PALESTINE FREEDOM PALESTINE FREEDOM PALESTINE 224 Postharvest Diseases of Fruits and Vegetables grow on insoluble material consisting primarily of cell-wall material obtained following the extraction of freeze-dried mycelium. In order to act as effective biocontrol agents and to qualify for development for commercial use on harvested crops, the antagonists should meet certain criteria. Moreover, determining the features needed for an antagonist to serve as a successful biocontrol agent is the first step before selecting potential antagonists. Wilson and Wisniewski (1989) listed the desirable characteristics of a potential antagonist: (1) genetically stable; (2) effective at low concentrations; (3) simple nutrient requirements; (4) capable of surviving adverse environmental conditions; (5) effective against a wide range of pathogens and on various fruits and vegetables; (6) resistant to pesticides; (7) a non-producer of metabolites deleterious to human health; and (8) non-pathogenic to the host. Various criteria connected with commercial development aspects, such as being easy to dispense and compatible with commercial handling practices, have also been mentioned. These desired features have guided many scientists in their biocontrol studies. In selecting an antagonist suitable for postharvest application, we need to look for those that are well adapted to survival and growth in wounds or on the produce surface under storage conditions, and that have an "adaptive advantage" over specific pathogens (Wilson and Wisniewski, 1989). For example, Rhizopus stolonifer is more sensitive to low temperatures than many microorganisms; therefore, an antagonist well adapted to low temperatures might prove advantageous against this pathogen. Another example is the advantage of a Candida oleophila strain in reducing the level of Penicillium expansum infection in nectarines under storage conditions: the effectiveness of the antagonistic yeast was not reduced by controlled atmosphere (CA) storage or by application of a commercial fungicide, therefore, it can be applied under CA conditions which are important for maintaining fruit quality, and in combination with dicloran which will, at the same time, prevent infection by other pathogens, such as Rhizopus (Lurie et al., 1995). For satisfactory control of postharvest pathogens of pome cultivars, the antagonist should be able to function under cold-storage conditions. Thus, the capability of several epiphytic bacteria isolated from apple leaves to control postharvest fungi on cold-stored apples, or that of a new strain of the yeast, Candida sake to control P. expansum, Botrytis cinerea, and R, stolonifer under various cold-storage conditions, is of importance when considering commercial application (Vinas et al., 1996; Sobiczewski et al., 1996). Since low temperatures and, in some cases. http://arab2000.forumpro.fr
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