Role of antibiosis in antagonism of Streptomyces hygroscopicus var. geldanus to Rhizoctonia solani in soil

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Abstract
S. hygroscopicus var. geldanus controlled rhizoctonia root rot of pea in previously sterilized soil if incubated for 2 or more days prior to infesting soil with R. solani and planting. S. hygroscopicus also reduced saprophytic growth and the population of R. solani in soil. Growth of R. solani was inhibited by geldanamycin, an antibiotic produced by S. hygroscopicus, on nutrient media. Methanol extracts of soils in which the antagonist was incubated for 2 or more days inhibited growth of R. solani. Geidanamycin concentration was 88 .mu.g per gram of soil after 7 days of incubation. Bioautography of soil extracts indicated that the inhibitory compounds were geldanamycin and 2 other compounds, also found in the geldanamycin standard. The period of incubation necessary for antibiotic production and disease control was similar, with no disease control occurring where no antibiotic was detected. Amending soil with geldanamycin, in amounts equivalent to that produced after 2 or 7 days of incubation, controlled disease and reduced saprophytic growth of the pathogen. Lesser amounts of the antibiotic did neither. No evidence for antagonism owing to competition or parasitism was found. S. hygroscopicus and geldanamycin also affected plant growth.