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Nearly 400 microorganism isolates, including bacteria, actinomycetes, and fungi, were collected from watermelon roots growing in soils suppressive and nonsuppressive to Fusarium wilt of watermelon. These isolates were screened for their ability to restore suppressiveness to microwave-treated suppressive soil and to reduce disease incidence in conducive field soil. Specific isolates of nonpathogenic Fusarium oxysporum from suppressive soil were the only organisms consistently effective in reducing disease (35 to 75% reduction) in both microwave-treated and natural field soils. Thus, we concluded that F. oxysporum was the primary antagonist responsible for suppression in this suppressive soil, although other organisms may contribute to suppressiveness. Selected isolates of F. oxysporum were effective in reducing disease when added to field soils at inoculum levels as low as 50 to 100 chlamydospores per g of soil, which was comparable to or below pathogen inoculum levels (100 to 200 CFU/ g of soil). Root colonization data indicated that reduction of disease was not directly related to the ability of the antagonist to colonize roots extensively or to reduce colonization by the pathogen. Effective antagonists were not associated with specific vegetative compatibility groups, indicating antagonists represent diverse isolates. In split-root experiments, in which the antagonist and the pathogen were physically separated from each other, root colonization by selected isolates of E oxysporum reduced disease incidence, verifying the mechanism of action as induced systemic resistance. Several isolates of F. oxysporum from this suppressive soil have potential for development as biocontrol agents.
Robert P. Larkin (Mon,) studied this question.