Fig. 1. Sprouts in a plant bed wilting from sclerotial blight. Some of the white mycelia of the fungal pathogen are visible on the soil near the base of the plants.
Fig. 2. Circular spot lesions on a sweet potato shortly after harvest. Some of the lesions have begun to peel away from the root as is common in storage.
Although they occur at different stages of cultivation of sweet potato, sclerotial blight and circular spot are caused by the same soilborne fungus, Sclerotium rolfsii, that also causes southern blight on many other crops.
Sclerotial blight occurs in the plant beds, but is much less common in Beauregard than other cultivars. Plants first show a bronze cast to the leaves, then wilt and may die (Fig. 1), usually within circular areas in the bed. When the weather is humid, a coarse white mycelium of the fungus may grow out on the soil surface around the base of the plants and up the lower part of the sprouts. Later, small sclerotia that resemble mustard seeds develop on the mycelia; these structures survive for years in the soil. Sclerotial blight is especially severe when there are ‘seed’ roots in the bed that are rotting because of some other disease, when there is dead leaf material on the surface such as when plastic covers have been left on too long, and when the plants are under stress from extremes of temperature or moisture.
Sclerotial blight can also be reduced by treating the ‘seed’ roots with dichloran (Botran), which reduces growth of the fungus on the surface of the seed roots before infection occurs.
Circular spot develops on storage roots shortly before harvest. Lesions can easily be confused with those of Streptomyces soil rot, but differ in that the margins of the lesions are usually sharply defined and circular, there is no identation of the root, the lesion is usually not cracked but may begin to peel off shortly after harvest, and the tissue is a lighter yellowish brown (Fig. 2). Other than choosing cultivars less prone to circular spot, there are few other proven measures available to reduce this disease.
Please direct questions or comments about sweet potato diseases to Chris Clark.
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