The fungus, Colletotrichum coccodes (syn. C. atramentarium), causing this disease, is considered a low-grade pathogen that attacks the potato plant when it is under stress. The disease is most frequently associated with light sandy soil, low nitrogen, high temperatures and poor soil drainage.
Symptoms vary from below-ground rot of roots, stems and stolons to above-ground wilting and yellowing of foliage. Foliage symptoms first occur at the top of the plant and later at mid and bottom regions. The symptoms may be confused with those caused by Verticillium wilt. Diseased areas on below-ground stems and stolons may also resemble those caused by Rhizoctonia spp. During the latter part of the growing season, as the lower portion of the stem tissue dries and is scaled away, an amethyst colour is common. The lower portion of the stem may also contain sclerotia (small hard black bodies about the size of a match head) both externally and internally. Much smaller sclerotia, barely visible to the naked eye, may develop on diseased areas of tubers in storage. These grayish areas may closely resemble silver scurf.
Control
Clean seed, crop rotation, adequate fertility and good irrigation management are recommended. Chemical controls are not normally used for this disease.