Black Dot

Black dot is a disease that affects roots, stems and tubers and is named for the small black, dot-like structures (sclerotia) that form on the surface of infected stems, stolons, and tubers. Black dot first appears in mid- to late season as a yellowing and wilting of plants. These symptoms are easily confused with Verticillium Wilt or early dying associated with bacterial soft rot. Wilting caused by black dot develops rapidly, in contrast to Verticillium wilt.

The black dot pathogen can cause severe decay of the cortical tissue of roots. Affected roots may appear stringy when pulled from the soil. In some cases, lesions on belowground stems and stolons may be confused with Rhizoctonia stem and stolon canker; however, black dot lesions are darker. In Tulelake, infected roots often lack dark lesions. The cortical tissue scales away, exposing the woody vascular tissue, which gives the roots a stringy, water-soaked appearance, amethyst in color. Small, dark, dotlike fungal structures (sclerotia) are almost always present near soil level with mature infections. Spore-forming structures (acervuli) with conspicuous hairs (setae) also are formed. Tubers infected with black dot appear to have a slightly darker or a rash-like appearance that closely resembles silver scurf. Presence of sclerotia distinguish black dot from silver scurf. Tuber blemishes are easy to see particularly when tubers are wet. Another common symptom of black dot is the adherence of stolons to the stem ends of tubers.

Black dot affects other plants in the potato family, including tomatoes, eggplants, and peppers. The fungus that causes black dot, Colletotrichum coccodes, survives as sclerotia, which form on tubers, stolons, roots, and stems at the end of the season. The pathogen can be introduced into clean fields via contaminated seed tubers, and disease levels increase in fields with a short cropping history between susceptible crops. Sclerotia can survive for long periods on plant debris in the soil. Spores produced in acervuli on aboveground plant parts can be spread by wind to other plants and cause infections if wounds are present.

The black dot pathogen often invades plants that are weakened by other diseases, and it may accelerate early death of vines infected with Verticillium, Pectobacterium (Erwinia), and possibly Phytophthora. Black dot occurs most frequently on plants grown in coarse-textured soils under conditions of low or excessively high nitrogen, high temperature, or poor soil drainage. Black dot infections are increased by windblown sand, which can cause abrasions that give the pathogen easy access.

Plant certified seed tubers, maintain adequate levels of nutrients, and avoid overirrigation. When fields become infested, rotate to nonhost crops such as grains for at least 3 years, and control potato volunteers and potato family weeds in the rotation crops. No potato cultivars are resistant, but early season cultivars may escape some damage in infested fields, and the tubers of russet cultivars are less severely affected than tubers of thinner-skinned cultivars.

Efficacy trials evaluating different fungicides have shown that although there are several fungicides available that can reduce root rot and foliage symptoms, none was able to significantly decrease the occurrence of tuber blemish.