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UC Pest Management Guidelines


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Potato

Black Dot

Pathogen: Colletotrichum coccodes

(Reviewed 8/07, updated 5/08)

In this Guideline:


SYMPTOMS AND SIGNS

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 bacterial early dying. Wilting caused by black dot develops rapidly, in contrast to Verticillium wilt.

The black dot pathogencan 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.

Black dot is distinguished by small, black, dotlike fungal structures (sclerotia) that form on the surface of infected stems, stolons, and tubers. 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. Another common symptom of black dot is the adherence of stolons to the stem ends of tubers.

COMMENTS ON THE DISEASE

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 is spread on contaminated seed tubers, and inoculum levels increase in fields continuously cropped to potatoes or related 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. Tubers of russet cultivars are less severely affected. Black dot infections are increased by the abrasion caused by windblown sand.

MANAGEMENT

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.

PUBLICATION

[UC Peer Reviewed]

UC IPM Pest Management Guidelines: Potato
UC ANR Publication 3463
Diseases
R. M. Davis, Plant Pathology, UC Davis
J. Nuñez, UC Cooperative Extension, Kern Co.
B. J. Aegerter, UC Cooperative Extension, San Joaquin Co.
Acknowledgment for contributions to the disease section:
C. Smart, Plant Pathology, UC Davis

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