Tubers infected with powdery scab develop … Tubers infected with powdery scab develop small purplish-brown pustules about 0.06 inch (1.5 mm) in diameter. The pustules typically become raised, brown, and wartlike as they enlarge to about 0.37 inch (9 mm) in diameter and rupture the tuber periderm. Powdery scab lesions on tubers may be confused with lesions of common scab, and laboratory confirmation of powdery scab is advised. Microscopic observation of mature powdery scab lesions typically reveals diagnostic dark brown spore balls of the causal fungus. In addition to tuber symptoms, S. subterranea causes galls on roots and stolons. Foliage symptoms have not been observed in California. toms have not been observed in California.