Potato scab complex diseases caused by Streptomyces spp. is very important which diminish crop quality, quantity and marketability. As pathogenicity genes in the causal bacterium are mobilized through a large mobile genetic element and cause the emergence of new pathogenic species, control of the disease is very critical. The scab causative bacterial strains were isolated from potato growing areas and they were characterized. They showed heterogenicity feathers belonged to S. scbies, S. acidiscabies, S. turgidiscabies and Streptomyces sp. Common potato scab-inducing strains produced thaxtomin but pitted lesion-inducing strains never produced this toxin nor hybridized to thaxtomin biosynthesis gene probe. PCR amplification of the sequences related to pathogenicity genes showed that only some tested strains harbor nec1 and txt gene sequences. For biocontrol of the causal agent of potato scab disease, 150 bacterial strains were isolated from the potato rhizosphere. Antagonistic activity of them towards the causative agent of scab disease were evaluated in a randomized design which 92 bacterial strains had antagonistic activity. Antagonistic strains belonged to Bacillus spp, Pseudomonas fluorescens, and P. putida . Results of greenhouse experiments carried out in a randomized block design showed significant differences among the treatments. The most effective strain reduced disease severity by 62.5% when applied as tuber coating and soil drenching. Strategy for selection and application of rhizobacteria antagonist to soil-borne pathogens, may increase the yield, reduce the disease control cost and protect the biological balance and resources and soil environment
