Potato [Solanum tuberosum] berry bioassay to detect Streptomyces spp. that produce phytotoxic thaxtomin A and cause potato common scab [streptomyces]

The thaxtomin-A-producing ability of Streptomyces spp. isolates (nine isolates of S. scabies, five isolates of S. turgidiscabies, one isolate of S. acidiscabies) was assessed by the development of necrotic lesions on potato berries. Berries collected from field-grown potato plants were inoculated with plugs of pathogenic isolates from starch nutrient or oatmeal media and then kept in a moist container for 7 days at 25 deg C under a 12-h photoperiod or continuous darkness. Each of the thaxtomin-A-producing, pathogenic strains caused an obvious necrotic lesion (more than 4-6 mm in diameter) on the berries. On the other hand, the nonpathogenic strains (two isolates of S. ipomoeae, one isolate of S. acidiscabies, six isolates of Streptomyces spp.) that did not produce thaxtomin A barely induced any necrotic symptoms. Crude chloroform extracts from the oatmeal-broth cultures of thaxtomin-A-producing strains of S. scabies, S. turgidiscabies and S. acidiscabies and the authentic thaxtomin A induced necrotic lesions on the berries. In contrast, crude extracts from strains that did not produce thaxtomin A and were nonpathogenic to potato tuber did not induce symptoms on the berries. The potato berry bioassay is thus a simple procedure that is suitable for the consistent detection of thaxtomin A production by Streptomyces spp. that cause common scab disease.