Two nonpathogenic isolates of Fusarium oxysporum were examined for their ability to counteract F. solani f.sp. pisi, which causes foot and dry root rot in pea. Antagonism was studied in vitro, in a sterilized field soil, and in six natural field soils. Besides native F. solani, other typical pea root rot pathogens occurred in the natural field soils. Both nonpathogenic F. oxysporum isolates reduced disease severity and prevented the plant weight losses that occurred owing to F. solani f.sp. pisi in sterilized soil. Precolonization of sterilized soil with nonpathogenic isolates increased the antagonistic effect. Also, in highly infested field soils the addition of nonpathogenic isolates resulted in lower disease severities and higher yields. Colonization of the soil organic matter by F. oxysporum reached 100% in sterilized soil, independent of the presence of F. solani, and 40 – 90% in naturally infested soils containing native F. solani. The performance of benomyl-resistant mutants of F. oxysporum did not differ from their wild types. Key words: antagonism, soil organic matter, colonization, Pisum sativum.
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