Technical Abstract: Three plant pathogenic species of Streptomyces, S. scabies, S. acidiscabies, and S. turgidiscabies, cause common scab of potato and other root crops. The disease symptoms are induced by a family of dipeptide phytotoxins called thaxtomins, which are produced by all three species. Thaxtomins are produced in vivo on diseased potato tissue and in vitro in oat-based culture media, but the regulation of thaxtomin biosynthesis is not understood. S. acidiscabies strain 84.104 was grown in a variety of media to assess the impact of medium components on thaxtomin A (ThxA) production. We found that ThxA biosynthesis is not correlated with bacterial biomass production, nor is it stimulated by a-solanine or a-chaconine, the two most prevalent glycoalkaloids in potato tissue. ThxA production was stimulated by oat bran broth, even after the bran had been extracted with hexane, ethyl acetate, methanol, and water, suggesting that oat bran contains one or more polymeric elicitors of ThxA biosynthesis. Oat bran contains high levels of xylans and glucans, and both of these carbohydrates, as well as xylans from wheat and tamarind, stimulated ThxA production, but not to the same extent as oat bran broth. Starches and simple sugars did not stimulate ThxA production. The data indicate that complex carbohydrates act as environmental signals to plant pathogenic Streptomyces, eliciting the production of thaxtomin and enabling the bacteria to colonize its host.