Diseases of Sweet Potatoes

Scurf (Monilochaetes infuscans)

Scurf is a fungal disease of sweetpotato trans-mitted from infected mother roots to transplants and then to the field. Scurf may persist in soils for several years. It normally affects the skin and outer layers of the storage root, but not the underlying flesh. Dark brown to black spots develop over the surface of the root throughout the growing season. The damage is mainly cosmetic, but scurf can increase water loss in storage. There are no above-ground symptoms. Controls include:

• using scurf-free seed roots,
• treating seed roots with an effective fungicide,
• bedding seed roots in soil free of the pathogen,
• cutting transplants at least 1 inch above the soil line,
• dipping slips in a fungicide if roots are infected,
• using a 3- to 4-year rotation.

Present levels of cultivar resistance are not sufficient for control.

Storage rots

Storage rots, including black rot, Rhizopus soft rot, charcoal rot, circular spot and Fusarium root rot represent a wide range of diseases. The best prevention measures are to add bleach to wash water, disinfect equipment, and cure roots promptly.

Soil rot or pox (Streptomyces ipomoea)

Soil pox is a bacterial disease common in the major sweetpotato production areas of the United States and Japan. Pox has become a serious problem in Eastern North Carolina within the last ten years, with more severe symptoms developing during seasons with hot, dry conditions.

Symptoms include a stunted feeder root system, black crusty lesions or 'scabs' (usually less than 1 inch in diameter) on roots and underground stems, and early girdling of storage roots resulting in mishapen, often dumbbell shaped roots. Infected plants are stunted, bronzed and chlorotic and may wilt or flower prematurely. Conditions leading to more serious disease loss include light, sandy soils, a pH greater than 5.2 and relatively dry soil. If periods of dry weather occur before roots start to form, yields are reduced, but few root lesions form. If the drought occurs after storage root initiation, there will be more lesions on the roots. Irrigation during dry periods may reduce disease severity but has not been found to increase yields.

Pox is very persistent, and heavily infested fields should not be used to produce sweetpotatoes unless there is a fairly high level of pox resistance in the cultivar being grown. Rotation out of sweetpotatoes for several years will reduce soil pox in the first year after sweetpotatoes are reintroduced into the field, but not in subsequent years. In lightly infested fields with low pH, fumigation may be sufficiently effective to produce a crop profitably. Resistant cultivars are listed in the Sweetpotato chapter. Soil pox spreads on infected soil or planting material, making it critical to sanitize planting and harvesting equipment between fields.

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