Scurf (fungus - Monilochaetes infuscans): Scurf is most severe in wet or poorly drained soils. Scurf appears as light brown blotches on the outside of roots. These areas may be small or run together to form large, irregular patches. Although superficial, only skin deep, infection reduces grade and causes undue shrinkage in storage. Avoid use of scurfy seed potatoes. Treat seed potato roots with a fungicide before bedding. Cut slips just above the soil line for transplanting. Practice rotations of three to five years. Avoid planting where organic matter in the soil is not well decomposed.
Root Knot (nematode - Meloidogyne sp.): Root cracking is often associated with severe root knot infection. Pitting and other surface blemishes are also noted. Vines may be stunted and show nutritional deficiencies. For diagnosis, split small, whip-like roots longitudinally and look for pearl-like swollen female nematodes embedded in the flesh. Often the area surrounding the nematode is dark. Nematodes may also be embedded in mature potato flesh but are more easily viewed for diagnostic purposes in smaller roots. Small galls or swellings can be detected on fibrous roots. Control options include crop rotation, resistant varieties, nematicide treatments and careful selection of seed potatoes. Cut slips above soil line to prevent transfer of infected roots to planting fields.
Stem Rot (fungus - Fusarium oxysporum f. sp. batatas): Young leaves at the tips of vines turn yellow while older leaves wilt then drop so that the center of the hill becomes bare. Infested slips may die soon after setting or become stunted and yellow. Stems at the soil line may turn slightly blue (blue-stem). The inner stem portion at or below the soil line becomes discolored with brown streaks in the vascular system. Selection of seed roots free of the disease is difficult because external symptoms are not always present. Resistant varieties have greatly diminished losses to stem rot.
Black Rot (fungus - Ceratocystis fimbriata): Circular, almost black spots appear on sweetpotato roots. Affected and adjacent areas have a bitter taste. Small, black lesions often completely girdle underground stems. The causal organism fruits abundantly in storage, which helps separate black rot from other storage rots. Once the most important sweetpotato disease, black rot is difficult to find.
Soil Rot (bacterium - Streptomyces ipomoea): Soil rot, also called pox, is a storage root and fibrous plant root disease. Storage roots are often misshapened with rough, scabby pits or shallow surface lesions that result in scars. Fibrous roots develop dark, necrotic lesions that eventually kill the root. Serious plant losses result when heavy infection occurs immediately after transplanting. Soil rot is favored as soil pH rises above 5.2. Crop rotation, resistant or tolerant varieties and soil fumigation are means of control. Soil rot is widespread and a major disease on sweetpotato.
Foot Rot (fungus - Plenodomus destruens): Foot rot is a relatively minor disease. On infected vines, the base of the stem turns brown at the soil line and leaves nearest the crown turn yellow and drop. It is most commonly observed from mid-season to harvest. Individual plants may produce few potatoes even though large vines develop during the season. The potatoes that are produced develop a firm brown rot at the stem end. Seldom is the entire root affected. Controls recommended for black rot and scurf will also control this disease. If foot rot is recognized to be a serious problem, then early harvest will aid in reducing losses.
Russet Crack and Internal Cork (virus - Sweetpotato feathery mottle virus): The virus is transmitted by aphids and it is believed all sweetpotatoes are infected. Symptoms, however, vary depending on strains of the virus and variety. Russet Crack is associated with Jersey-type sweetpotatoes. Small, longitudinal cracks encircle parts of storage rots. Internal cork was once a serious problem, especially in the Puerto Rico variety. Brown to black corky areas develop in the flesh of roots and can go undetected until sliced. Modern day varieties are resistant to these two symptoms. Foliar symptoms vary from purple ring spots to vein clearing.
Bacterial Stem and Root Rot (bacterium - Erwinia chrysanthemi): More common in storage than in fields. Infected areas inside potatoes are light brown and watery. Plants show dark brown to black lesions on stems with dark streaking in the vascular tissue. Storage root infection and vine infections often occur independent of each other. The bacterium is favored by high temperatures and humid conditions. Some varieties are more susceptible than others. Control: Reduce wounding, select clean seed potatoes, and maintain adequate gas exchange while plant beds are covered.
Circular Spot and Sclerotial Blight (fungus - Sclerotium rolfsii): Although caused by the same organism these two diseases are completely different. Sclerotial blight is a seed bed disease. The white, coarse fungus grows wildly on bedded roots, causing sprouted slips to die. It is easily identified by sclerotia that develop in the fungal growth. Sclerotia are balled up strands of the fungus, about the size of mustard seed, that serve a survival function. They are white at first but progress to a dark brown color. No fungal structures are observed with circular spot which is a very common problem on storage roots. Lesions are round and up to one inch in diameter. As potatoes dry in storage, the skin in the center of the spot may crack. Circular spots seldom progress in size in storage. There is no chemical control available. Crop rotation is advised. Early harvest may reduce incidence since infection seems greater later in the growing season.
Storage Rots (fungi - soft rot - Rhizopus stolonifer, Java black rot - Diplodia gossypina, charcoal rot - Macrophomina phaseolina and others): Storage rot losses are greatly reduced when disease control practices are followed that yield high quality sweetpotatoes from the field. Some fungi causing storage rots infect roots in the field before harvest while others enter the potatoes through wounds made at harvest or during handling. Decayed spots may be dry or soft, the latter due principally to Rhizopus rot. Sweetpotatoes should be cured for seven to 10 days at 85oF. and at 90 percent relative humidity before being stored. Curing allows a natural healing process to take place. Potatoes should be stored at 55o to 60oF. and at 85 to 90 percent relative humidity. Avoid rough handling between curing and storage because additional wounding may occur. Store in a thoroughly clean and disinfected room with adequate ventilation. Do not permit the storage temperature to drop below 55oF. or injury will occur. For control of Rhizopus rot during the processing-to-market interval, apply a fungicide spray immediately after washing, when potatoes are on conveyor belts or rollers.
Cold Damage (physiological): Pithy, dark discolorations will appear internally on sweetpotatoes subjected to low temperatures. Temperatures do not have to reach the freezing point to cause damage. Sweetpotato is a tropical plant and will suffer injury at temperatures below 55oF.
Growth Cracks (physiological): Scientists have not yet determined the conditions that cause cracking of sweetpotatoes. In some varieties, it appears to be an inherited characteristic. Rotation reduces incidence of growth cracks.
Cotton Root Rot (fungus - Phymatotrichum omnivorum): See section on cotton root rot.
Root Knot Fusarium Internal Sclerotial
Variety Nematode Wilt Cork Blight Soil Rot
Jewel R R R I S Centennial S I I S S Puerto Rico I-S S S I S Jasper I-R I-R R S R Gold Rush S R S - S Nemagold R S S - I-R Redgold S - I - S Topaz R R R R S Beauregard S R R I R-I Hernandez I-R I-R R - I-R Darby S R R - I-R Excel R R R I S-I