Surface rot of sweet potatoes, caused by the fungus Fusarium solani, may occur in the field but is most commonly a storage disease. It sometimes causes greater losses than do all other storage rots.

Surface rot lesions are generally circular, with regular margins, and the affected area is brown in color. The lesions may develop to 2 or more inches in diameter. They develop slowly, taking 6 weeks or more to become evident. The diseased area becomes sunken in older tissues. This disease, as its name implies, causes shallow lesions that usually do not extend deeply into the flesh. Other invading organisms, including soft rot, may follow, however, and cause the entire root to decay. The inner color of surface rot lesions is brown and older lesions may become dark brown. Sweet potatoes affected with surface rot become shrunken due to loss of water and finally become hard and mummified.

The symptoms of surface rot are distinctive, and this disease need not be confused with black rot or the bruises. External and internal color of black rot lesions is black and not brown. The black rot organism penetrates deeply into the infected sweet potato instead of shallowly as does the surface rot fungus.

CONTROL

Surface rot becomes most prevalent when sweet potatoes are harvested during wet, cold weather and cannot be dried before being stored. The disease becomes most destructive when there is inadequate ventilation and when storage temperatures are kept too cold. When the temperature at harvest time is cold and the roots cannot be dried, wet, soil-covered sweet potatoes should be placed in heated, well-ventilated storage on the day they are harvested. The roots should be stored in ventilated containers where the air can circulate and the roots can dry quickly. Decay of the roots becomes most serious at the bottom of containers with solid sides in which air circulation is poor.

Surface rot is likely to become most prevalent near the floor along outside walls of the storage house where inadequate heating occurs. Effective wall insulation should be provided and provisions should be made that good air circulation occurs in this area.

Web updated 9/01/06
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