BLACK DOT (vine and tuber)

Colletotrichum coccodes causes the disease black dot; it is a weak pathogen considered a minor disease in potatoes. The incidence of the disease is not known since it is often associated with other wilts on the vine and silver scurf on tubers.

I. On vines, symptoms are first visible in mid to late summer as yellowing and wilting of foliage at the tops of the plants, similar to that of Fusarium and Verticillium wilts. Plants will turn brown and die. Currently, black dot is considered part of the “early dying complex” which includes other wilts and early blight.

Numerous tiny (1/50 inch) black “dots” may appear on the stem. These are sclerotia, dormant fungal masses, that appear as small black sea urchins under magnification. The most striking symptom appears below ground on the stem. When the outer tissue (cortical scales) are peeled back, the exposed woody vascular tissue turns reddish or amethyst color. The fungus produces the black dots on both the inner and outer surfaces of the stem. Necrotic lesions occur on the underground stems, roots, and stolons, and pinhead-sized black sclerotia, the dots, can be seen on the affected areas. Severe rotting of underground stems and roots may result in early death of the plant and reduction in tuber size. The symptoms -- black dots, amethyst color and outer cell sloughing -- may also be seen on stolons. Remnants of infected stolons still attached to tubers aid in disease identification.

Black Dot is introduced into the soil on infected seed tubers. Although infection is associated with the pathogen in the soil, the pathogen can also be spread by wind and may be associated with sand blasting of leaves and stem.

Black dot is often associated with sandy soils with low nitrogen levels. The disease may be favored by high temperatures and heavy irrigation in some poorly drained fields. Black dot attacks aging tissue, or injured or stressed plants. Heat stress was a common problem. Black dot can also overwinter in debris left in the field. There are no resistant cultivars. Crop rotation especially with grains, clean seed and fields, and good irrigation and fertility management may be helpful. Tomato, pepper, and eggplant also are hosts. There are no chemical means for control of black dot.

II. For tuber symptoms, refer to black dot under TUBER BLEMISHES

Since black dot develops during storage in combination with other pathogens causing similar symptoms, it is difficult to visually identify. The development of the fungus sclerotia on the tuber surface causes only shallow markings but the appearance may be of importance in marketing the product. Black dot may colonize the skin of new tubers causing grayish areas very similar to silver scurf. The black sclerotia “dot” can readily be seen with a hand lens. Thin skinned potato cultivars are much more susceptible to black dot than thicker skinned ones.

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