Antagonist Selection

The first successful attempts in BCPD of fruits, which stimulated research in postharvest biological control, used soil isolated bacterium, Bacillus subtilis, to control brown rot of stone fruits caused by M. fructigena (Pusey and Wilson 1984). Subsequent works focused on screening natural microflora from the aerial surfaces of apple and pear trees for antagonistic activity against decays caused by P. expansum and B. cinerea (Janisiewicz 1987). This resulted in the isolation of many bacteria and yeasts that were effective in controlling fruit decays caused by these pathogens. Isolation from the fruit surfaces has become a standard practice and is the most efficient source of antagonists against postharvest fruit pathogens of temperate, subtropical, and tropical fruits (Adikaram and Karunaratne 1998; Arras 1993; Chalutz et al. 1988; Chand-Goyal and Spotts 1996; Droby et al. 1999; Guinebretiere et al. 2000; Huang et al. 1992; Kanapathipillai and bte Jantan 1985; Lima et al. 1998; Qing and Shiping 2000; Teixido et al. 1998a; Testoni et al. 1993; Zahavi et al. 2000). A variety of enrichment procedures, employing either fruit juice or tissue, have been used to isolate microorganisms best suited to colonize wounded fruit tissue (Janisiewicz 1991; 1996; Wilson et al. 1993). The enrichment procedures appear to favor isolation of the resident fruit microflora, with the yeasts being isolated most frequently, followed by bacteria. Filamentous fungi have been isolated only sporadically (Janisiewicz 1996; Wilson et al. 1993). The rapid colonization of wounds by yeasts is necessary for preemptive exclusion of the wound-invading pathogens. The number of species that are residents on a specific kind of fruit is limited, and reports from various laboratories worldwide increasingly describe biocontrol potential of the same antagonist species isolated at different locations (Chand-Goyal and Spotts 1996; 1997; Falconi and Mendgen 1994; Ippolito et al. 2000; Janisiewicz et al. 1994; 2001; Leibinger et al. 1997; Lima et al. 1998; McLaughlin et al. 1992; Roberts 1990; Wisniewski et al. 1988). Recent studies; however, indicate great diversity within an antagonist species, even at a single geographical location, with regard to effectiveness in controlling fruit decays and other factors important in commercializing a biocontrol agent (Janisiewicz et al. 2001; Schena et al. 1999). Thus, investigating the same species of the antagonist at various locations may lead to finding an antagonist with superior attributes. An effective antagonist may also be found by screening starter cultures used for food products (Pusey 1991), various culture collections (Filonow et al. 1996), and even by exploring an aquatic environment, as is the case with bacteriophages used against soft rotting bacteria (Eayre et al. 1995).

In addition to being effective in controlling fruit decays, antagonists should have certain attributes to make them good candidates for commercialization. These include: compatibility with postharvest practices (storage temperatures, relative humidity-RH, storage atmosphere with elevated CO2 and reduced O2, handling in water, heat drying tunnels, etc.), treatments and additives (waxes, antioxidants, flotation salts), ability to grow efficiently in a commercially used media for mass production, ease of formulation, and the lack of potentially deleterious effects on human health that would disqualify them from being approved by regulatory agencies. Human safety, in particular, necessitates a thorough approach in identifying an antagonist. Misidentification may result in abandonment of commercial development of the antagonist, and, if not detected early, may be very costly.

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