Biocontrol of Fusarium Wilt of Cucumber Resulting from Interactions BetweenPseudomonas putidaand Nonpathogenic Isolates ofFusarium oxysporum

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Abstract
Fluorescent pseudomonads and nonpathogenic isolates of Fusarium oxysporum were effective in inducing suppressiveness to Fusarium wilt of cucumber when added to soil together (pH 6.7) but ineffective when added separately. Suppressiveness by such combination treatments was enhanced in nearly neutral (pH 6.7) to alkaline soils (pH 8.1), in comparison with acid soil (pH 5.5). Strains of fluorescent pseudomonads reduced the germination of chlamydospores of nonpathogenic and pathogenic isolates of F. oxysporum in the rhizospheres of cucumber plants. Population densities of fluorescent pseudomonads increased significantly in the rhizosphere of cucumber in the presence of a nonpathogenic isolate of F. oxysporum in soil of pH 8.1. It is hypothesized that the activity of fluorescent pseudomonads and their siderophore production are enhanced by increased root exudates induced by relatively high population densities of nonpathogenic isolates of F. oxysporum. This, in turn, leads to competition for iron, which is essential for successful germination of the pathogen and penetration of the host.