Antagonism of Streptomyces Hygroscopicus Var. Geldanus to Rhizoctonia Solani

118 p.Thesis (Ph.D.)--University of Illinois at Urbana-Champaign, 1982.Antagonism between soil microorganisms is a common phenomenon. However, little is known of the mechanisms by which antagonism occurs. This investigation was designed to understand the nature of antagonism between Streptomyces hygroscopicus var. geldanus and Rhizoctonia solani and the mechanism by which antagonism occurs.S. hygroscopicus controlled Rhizoctonia root rot of pea in previously sterilized soil if incubated for seven days prior to infesting soil with the pathogen and planting. Disease control was not found without prior incubation. Disease was not controlled in unsterilized soil in which the antagonist was incubated for seven days. In addition to controlling Rhizoctonia root rot, S. hygroscopicus reduced both saprophytic growth of the pathogen and the population of the pathogen in soil.S. hygroscopicus is reported to produce the antifungal antibiotic geldanamycin. R. solani was inhibited by geldanamycin at a concentration as low as 0.5(mu)g/ml on nutrient media, with nearly complete inhibition at 50(mu)g/ml. Methanol extracts of soils in which the antagonist was incubated for seven days inhibited R. solani. The amount of inhibition was equivalent to 88(mu)g of geldanamycin per gram of soil, based on an extraction efficiency of 60%. No inhibitory substance was detected when soil was extracted shortly after adding the antagonist. Bioautography following thin-layer chromatography of soil extracts indicated that the inhibitory compounds were geldanamycin and two other compounds found in the geldanamycin standard. The period of incubation necessary for antibiotic production and disease control were similar, with no disease control occurring in the situation where no antibiotic was detected. Amending soil with geldanamycin, in amounts equivalent to that produced after two or seven days of incubation, controlled disease and reduced saprophytic growth of the pathogen. Lesser amounts of the antibiotic did neither. A slight discoloration of below ground plant parts and reduced plant growth was found with soil in which S. hygroscopicus was incubated for seven days or soil amended with geldanamycin. Antagonism was not due to competition (nitrogen, carbon, or space) or parasitism.Thus, antagonism of S. hygroscopicus var. geldanus to R. solani is apparently due to the production of geldanamycin in soil.U of I OnlyRestricted to the U of I community idenfinitely during batch ingest of legacy ETD