Identification and Detection of a Virulence Gene of Streptomyces scabies Causing Potato Scab in Thailand

Potato scab is caused by several pathogenic Streptomyces species which diminish crop quality, quantity and marketability. In the present study, a 16S rRNA gene sequence was used to detect Streptomyces species in potato tuber lesions harvested in Chiang Mai, Thailand and to evaluate a virulence gene as a reliable marker for the detection of pathogenic Streptomyces species by PCR assays. Streptomyces isolates were isolated from potato scab lesions, of which one isolate was pathogenic on potato tubers. The pathogenic isolate MJ21 was identified as Streptomyces scabies based on 16S rRNA gene sequence and morphological characteristics. Subsequently, isolate MJ21 produced PCR products from the tomA and txtAB genes, which are related to the production of tomatinase enzyme and thaxtomin A, respectively. Moreover, when grown on nutrient agar (NA) with MJ21, eggplant seedlings showed severe stunting of the roots and shoots, and failed to germinate; by comparison, seedlings/seeds grown on NA plates without MJ21 exhibited no symptoms. This study reports that S. scabies MJ21 has a toxigenic region (TR) that is associated with the tomA and txtAB genes. Keywords: Pathogenicity tests, Potato scab, Streptomyces scabies, Toxigenic region, Virulence gene