Diversity of Streptomyces strains causing potato scab disease in Hamedan province and their thaxtomin production potential

Hamedan province is one of the main potato growing areas in Iran. Potaoto scab disease caused by Streptomyces spp. is one of the prevalent and important diseases of potato in this provice. Potato tubers with symptoms of scab diseases including raised and deep or shallow pitted lesion were collected from potato growing area in this region. A total of 20 and 25 strains of Streptomyces were isolated from raised and pitted lesions on potato tubers respectively. The pathogenicity of these strains was confirmed on potato mini tubers and mature plants under greenhouse condition. Different isolated scab inducing Streptomyces strains showed diverse of symptoms on potato tubers. Protein electrophoretic profiles of the Streptomyces indicated diversity among the isolated strains. Colony color of the isolated Streptomyces strains on YMEA medium was grey and white-grey and they formed spiral or flexous spore chain on their aerial mycelium five to 10 days after cultivation. All selected strains were belonged to the Streptomyces genus and most of them used D-fructose, manitol, D-glucose, raffinose, rhamnose, sucrose, D-xylose, meso-inusitol and arabinose as a sole source of carbon. Phenotypic characteristics of the tested strains showed that they were belong to Streptomyces and includs three groups. First group of the strains produced melanin pigment from tyrosine and they were identified as S. scabies. Strains of the second and third groups were identified as S. acidiscabies, and Streptomyces sp. respectively.
Untill now three phytotoxins including thaxtomine (the main pathogenicity factor), concanamycine and FD-891 compound were reported as pathogenicity factor of the Streptomyces strains inducing potato scab disease all over the world. All of these phytotoxins induce raised, russeted or neted lesion on potato tubers. Phytotoxins were extracted from culture filterate of the tested Streptomyces strains grown in oatmeal broth using aseton. The extracted phytotoxins were concentrated by rotary evaporator and were subjected to thin layer chromatography (TLC). Results showed that only raised, russeted or neted lesion inducing strains including S. scabies EF-35 produced thaxtomin. Potato pitted inducing Streptomyces strains did not produced thaxtomin or concanamycin as pathogenicity factors however they produced an unknown compound which maybe a new pathogenicity factor. Characterization of this new pathogenicity factor is very important for management of the disease in the future.