Summary: Phytotoxins with potato-scab-inducing activity were produced by the pathogenic strain RB2 when grown on oatmeal agar medium or in oatmeal broth medium. Five compounds isolated from cell filtrates were identified as thaxtomin compounds 1-5, previously reported as produced by grown on potato slices. The optimum temperature for phytotoxin production in oatmeal broth medium was 28 °C. Production of thaxtomin A, the major product, was repressed at least 130-fold when RB2 was grown in oatmeal broth medium supplemented with 0.5% glucose. Thaxtomin A production was also repressed by tryptophan and tyrosine, precursors of which may be involved in feedback inhibition of early steps in biosynthesis. Phytotoxins were secreted by the organism when the cells reached late exponential to early stationary phases of culture growth. The time during growth at which each thaxtomin compound was produced suggests a pathway for the latter steps in thaxtomin biosynthesis.


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