Selection and characterization of strains of Streptomyces suppressive to the potato scab pathogen

Ninety-three strains of Streptomyces were isolated from lenticels of potato tubers grown in naturally disease-suppressive and disease-conducive soils. Twenty-two strains showed more antibiotic activity against virulent Streptomyces scabies RB3II than the standard pathogen-suppressive strains PonR and PonSSII. These 22 suppressive strains were nonpathogenic on leaf-bud tubers in the greenhouse. There suppressive strains plus standard strains PonR and PonSSII, 4 Minnesota (MN) virulent S. scabies strains, 11 virulent S. scabies strains, and 2 virulent Streptomyces acidiscabies strains from the eastern United States were evaluated in all possible paired combinations for antibiotic activity and competitive interactions in antibiotic and co-plating assays. A significant positive correlation between the ability of a strain to inhibit others and the ability of that strain to resist inhibition was observed in antibiotic assays (r = 0.6, P < 0.01). On average, suppressive strains inhibited other strains more and were less inhibited by other strains than virulent strains. Suppressive strains significantly reduced scab and did not affect tuber yield in a field-pot test. Taxonomic tests were used to characterize the Streptomyces strains used in this study. All the virulent strains and 54% of suppressive strains were classified as S. scabies.