INFLUENCE OF DISEASE-SUPPRESSIVE STRAINS OF STREPTOMYCES ON THE NATIVE STREPTOMYCES COMMUNITY IN SOIL AS DETERMINED BY THE ANALYSIS OF CELLULAR FATTY-ACIDS

Analysis of cellular fatty acid profiles was used to distinguish among introduced pathogen- suppressive strains and indigenous strains of St reptomyces spp. isolated from soil of field plots established to test the efficacy of Streptomyces strains PonSSII and PonR in the biologica l control of potato scab. Reference libraries of fatty acid profiles w ere developed for a collection of known pathogenic strains and the int roduced suppressive strains. Population densities of pathogen-related, suppressive, and saprophytic Streptomyces strains were determined fro m the relationship of field isolates to mean library profiles using cl uster analysis and the unweighted pair-group method using arithmetic a verages. Community diversity was similarly determined. Streptomyces st rains PonSSII and PonR were distinguished from each other and from the pathogen group (which clustered together) based on fatty acid profile s. The introduced, suppressive strains successfully colonized the soil and represented 2-19% of the isolates sampled over 2 years. The intro duction of the suppressive strains inhibited the population of strains related to the pathogen library at each sample date; the pathogen pop ulation was substantially lower in soil from treatments where the supp ressive strains were introduced compared with the nonamended control. At harvest, the pathogen-related population was suppressed 85-93 and 3 6-44% in 1991 and 1992, respectively, in treatments with the suppressi ve strains compared with the nonamended control. Diversity of the comm unity was not affected by the introduced strains, and diversity and eq uitability indices were similar among treatments at any sample time. T he inhibition of the pathogen-related population was correlated with a reduction of scab symptoms observed in the field plots into which the suppressive strains were introduced. Implications of a fundamental sh ift in the pathogen-related population in response to the introduction of the suppressive strains for long-term biological control of potato scab are encouraging.