Biological control of potato scab and antibiosis by antagonistic Streptomyces sp WoRs-501

Streptomyces sp. isolate WoRs-501 suppressed potato scab caused by S. turgidiscabies and S. scabiei in a field pot trial. An inoculum of 2 x 10(7) colony-forming units (CFU)/g dry soil was sufficient to establish a population with a stable suppressive effect on the disease. In the field, mixing the WoRs-501 inoculum (approximately 10(9) CFU/g dry mass) at a ratio of 1:9 (v/v) with scab-infested field soil at 30 x 30 x 15 cm per plant decreased the disease severity score by 26-54 % in comparison with the untreated control. To understand establishment of WoRs-501 in soil and the mechanism of the suppressive effect of the isolate on potato scab, we analyzed the population dynamics of WoRs-501 and scab pathogen in soil using real-time quantitative competitive quenching probe PCR. In a pot trial using soil that was initially treated with 2 x 10(7) WoRs-501 CFU/g dry soil, the copy number of the WoRs-501 16S-23S rDNA ITS sequence was maintained approximately 10(7) copies/g dry soil during potato cultivation. Treatment with 2 x 10(7) WoRs-501 CFU/g dry soil reduced the copy number of the scab pathogen nec1 gene in infested soil below the detection threshold. Though the copy number of this gene increased gradually during potato cultivation, the nec1 copy number remained at a significantly lower value than in the untreated control. An extract of the WoRs-501 inoculum inhibited mycelial growth of S. turgidiscabies in vitro. Therefore, antibiotics from WoRs-501 decrease scab pathogen populations in soil early in potato cultivation and strongly suppress scab disease.