Several Streptomyces species are reported from the potato growing area in the world which cause potato scab diseases. Pathogenicity genes of them are nec1 and thaxtomin biosynthesis genes including txtA, txtB, txtC and txtD and the main produced phytotoxins are thaxtomin, concanamycin and a compound named as FD-981. As potato scab disease is an important disease in Iran, samples which shown scab symptoms including raised, netted and deep or shallow-pitted lesions collected from the main potato fields. The causative agent Streptomyces strains were isolated, which they were very heterogeneous belong to four main groups: S. scbies, S. acidiscabies, S. turgidiscabies and Streptomyces sp. They were pathogenic on potato, parsnip, horseradish, carrot and some other tested plants. Raised and netted potato scab-inducing strains produced thaxtomin determined by thin layer chromatography, but this phytotoxin could not be detected in the pitted lesion-inducing strains. Selected strains were examined for the presence and situation of the pathogenicity related genes as they induced variable disease symptoms under field and greenhouse condition. Pulsed field gel electrophoresis technique revealed that most of the tested strains carried a linear plasmid. Amplification of the pathogenicity gene fragments and Southern hybridization analysis showed that only some tested trains harbor nec1 and txt genes. A total of 20 representative strains was grown in modified oatmeal medium and extracted with ethyl acetate. It was shown that some strains produced pathogenicity compound (s) other than thaxtomin which induced pitted lesion on potato tuber slice.
