Various mechanisms of biocontrol have been suggested for antagonists effective in BCPD of fruits and often more than one mechanism was implicated for a single antagonist. In no case, however, was the biocontrol mechanism fully explained. The putative mechanisms included competition for limiting nutrients and space, lysis, induced resistance, direct parasitism, and production of inhibitory substances. Attachment of antagonists to a fungal hyphae was observed in some antagonist-pathogen interactions, but its role remains largely speculative (Arras et al. 1998; Wisniewski et al. 1989; 1991). The main reasons for the limited knowledge in mechanism of biocontrol have been a lack of appropriate methods to study microbial interactions in wounds of fruit, and the fact that progress in BCPD was driven by advances in microbial ecology of the antagonists. However, recent advances in microbial sensing of nutrients on plants (Lindow et al. 2002), molecular approaches (Bassett and Janisiewicz 2001; Jijakli et al. 2001; Jones and Prusky 2002; Yehuda et al. 2001), and a method allowing separation of competition for nutrient and space using natural substrates (Janisiewicz et al. 2000) may lead to better explanation of the significance of various biocontrol mechanisms. Progress in microbial ecology of the antagonists led the commercialization of BCPD of fruits in a relatively short period of time, but further expansion will greatly depend on achieving the full potential of BCPD, for which knowledge of the mechanisms of biocontrol will be essential.
Bacterial antagonists such as Bacillus spp. (Gueldner et al. 1988) or Pseudomonas spp. (Bull et al. 1998; Janisiewicz and Roitman 1988) produce a variety of antifungal compounds in artificial media, which by themselves can provide effective control of postharvest decays of fruits (Bull et al. 1998; Janisiewicz et al. 1991; Takeda and Janisiewicz 1991). But the role of these antifungal compounds as the biological control mechanism is uncertain, because they either can not be detected in fruit wounds after inoculation with the antagonist (Bull, personal communication), or pathogen mutants resistant to these inhibitory substances are still controlled by the antagonists (Smilanick, personal communication).
Yeast antagonist such as Pichia anomala strain K (Gravesse et al. 1998; Jijakli and Lepoivre 1998), P. guilliermondii (Arras et al. 1998; Wisniewski et al. 1991), or yeast like Aureobasidum pullulans (Castoria et al. 2001) effective in controlling gray mold of apples, produce ß-1,3-glucanase, which caused lysis of the B. cinerea hyphae. Production of this enzyme by P. anomala strain K was stimulated in the presence of cell wall preparations of B. cinerea in apple wounds resulting in improved biocontrol (Gravesse et al. 1998; Jijakli and Lepoivre 1998). This enzyme also increased in apple wounds treated with the A. pullulans cells, but how much of this increase could be attributed to production by the antagonist, on the fruit itself, was not resolved (Castoria et al. 2001). This antagonist can also produce aurebasidins, antibiotics whose role has not been determined. The antagonistic yeast, Candida saitoana, effective in reducing decays of citrus and apple, induced chitinase activity in apple (Wilson and El Ghaouth 1993). C. oleophila, used in the commercial product Aspire, induced resistance responses such as production of chitinase, b-1,3-endoglucanases, PAL, phytoalexins scoparone and scopoletin and ethylene in flavedo tissue of grapefruit (Droby et al. 2002). The contribution of these induced resistance responses to biocontrol was not determined. The yeast C. famata, effective in reducing green mold caused by P. digitatum on oranges, increased the phytoalexins scopoletin and scoparone 12-fold in fruit wounds after four days, but the role scoparone in biocontrol is uncertain due to its relatively slow production (Arras 1996). The antagonists Cryptococcus laurentii and Sporobolomyces roseus, effective against gray mold of apple, utilized the apple volatile, butyl acetate, which stimulated germination and adhesion to membranes of B. cinera conidia (Filonow 1999; 2001). The significance of these phenomena in the biological control was not established due to the technical difficulties in conducting this type of experiment in fruit wounds. When these antagonists were applied to harvested fruits, they colonized fruit wounds rapidly, and competition for limiting nutrients and space was suggested as an important biocontrol mechanism. Removal of limiting nutrients may also be responsible for maintaining the dormancy of Colletotrichum spp. appresoria on fruit treated with antagonistic Bacillus spp. (Korsten and Jeffries 2000).
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