Production of thaxtomin A by two species of Streptomyces causing potato scab.

A total of nine isolates of streptomycetes were isolated from scab lesions on potato tubers. Five of nine isolates were pathogenic on potato minitubers. Four of the pathogenic isolates produced thaxtomin A (ThxA) in infected tuber tissues. The lesion surface areas inducing ThxA were highest in treatment of the minitubers with an extract of OMB inoculated with S-66 and S-67, intermediate with that inoculated with S-64 and lowest with S-63. The pathogenic isolates were identified by gray aerial mycelia, melanin pigment productivity, the type of spore chain morphology and carbon utilization as S. scabies strains S-63, S-64 and S-68, and S. acidiscabies strains S-66 and S-67. Strains S-63 and S-64 produced 0.65 and 1.60 mg ThxA per L of OMB, respectively, strains S-66 and S-67 producing similar amounts, viz. 2.36 and 2.10 mg/L, respectively. The optimal temperature for production (by both species) was 28 degrees C. Production of ThxA by S. scabies strain S-64 and S. acidiscabies strain S-66 was suppressed at least 50-fold at 0.5 and 0.3% of glucose, respectively. Fructose enhanced the production by both species.