Small, bumpy, irregular black lesions that look like dark soil on potato tuber skin are the classic sign of this fungal disease. Caused by the fungus rhizoctonia, black scurf generally occurs near the end of the growing season. Black scurf lesions do not wash off, but can be scraped off with a fingernail. Scurf is largely a cosmetic problem and rarely affects the eating quality of the potato. However, if the infection is severe, tubers may be small and misshapen.
Early in the growing season, the same fungus may attack the stems of potato plants at or just below the soil line (in which case the disease is called rhizoctonia canker). A powdery white mold may also be visible at the soil surface. Black scurf is different from potato scab, which appears as larger corky brown lesions. The fungus can survive in the soil for many years, even without a host plant present.
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