Differential production of thaxtomins by pathogenic Streptomyces species in vitro.

Streptomyces scabies and S. acidiscabies, causal agents of potato scab, produced the phytotoxin thaxtomin A and, to a lesser extent, other thaxtomins in oatmeal broth (OMB). Two nonpathogenic strains, Streptomyces lividans TK 24 and Streptomyces sp. 84-05, did not produce thaxtomins in OMB. Though thaxtomin A was produced by S. scabies strain 87-22 in potato broth, fresh potato starch, and some commercial starch preparations, production was much greater in OMB than in other media tested. Thaxtomin A was not produced in Luria broth or tryptic soy broth. Production of thaxtomin A by 87-22 was suppressed by 0.5 % glucose in OMB but was stimulated by up to 5.0 % glucose in oatmeal agar. Streptomyces scabies strain 87-22 produced 4.25 μg thaxtomin A per milliliter of OMB while strain 84-34 produced 0.17 μg per milliliter of OMB. Streptomyces acidiscabies strains 84-110 and 90-25 produced similar amounts of thaxtomin A, 2.65 and 4.45 μg per milliliter of OMB, respectively. Strain 87-22 was much more virulent on tubers of Chippewa, a scab-susceptible potato cultivar, than was strain 84-34. A tuber slice bioassay was useful for detection of thaxtomins in culture media and for identifying pathogenic Streptomyces strains