No symptoms are visible above ground. Infected tubers are misshapen, or have warty swellings on the surface, and are hard. The whole or part of the tuber may be infected. Numerous brown-black specks, interspersed with lighter brown specks, occur in the flesh. The specks (spore sori) are about 1 mm in diameter and are filled with rusty brown spore balls. Infected tubers later become a dry brown powdery mass containing numerous spores. Galls resembling deformed tubers arise on the stems or stolons underground.