Scurf occurs only on the underground portions of the sweet potato plant. The fungus does not penetrate deeply into the tissues but invades and kills the outermost layers of cells and causes a brownish discoloration of infected areas. Individual infections vary in size but remain relatively small. Many scurf infections may occur on the same root; when numerous, they completely cover the infected area.

The scurf organism may attack growing sweet potato plants in the hotbed or field. Scurf infections are especially conspicuous after sweet potatoes have been cleaned for marketing. Infected roots are unattractive and therefore sell for less. The diseased areas may increase in extent after sweet potatoes are placed in storage, but new infections on stored roots are rare. Affected roots shrivel during storage and become dry and leathery because the protective cork layer of the skin has been destroyed.

DISEASE CYCLE

The fungus survives in soil from one season to the next. The disease is most severe during rainy seasons and in low, wet soils or soils containing large quantities of organic matter.

CONTROL

Sanitation is very important for the control of scurf. Plant only certified, disease-free roots or slips. All hotbeds should be disinfected with steam, methyl bromide, vapam, chloropicrin, or some other type of fumigant before planting. Slips should receive a fungicide dip prior to planting. The fungicide thiabendazole (Mertect 340-F) is labeled for several diseases of sweet potato as a preplant dip. Seed roots should be treated for 1 to 2 minutes, then planted. Cultural practices in the field, including weed control, insect management, and proper irrigation will help suppress disease development. Handling sweet potatoes at harvest should be done with care to avoid bruises and mechanical injuries. Storage facilities should be thoroughly cleaned before harvest.

Do not apply stable manures on plant beds or on sweet potato fields. Do not plant sweet potatoes in fields where cattle fed on sweet potatoes have been allowed to graze. These practices may introduce the fungus to these areas and cause significant disease development. Crop rotations of at least 4 years are recommended.