Bacterial Stem and Root Rot (bacterium – Erwinia chrysanthemi): More common in storage than in fields. Infected areas inside potatoes are light brown and watery (See Photo). Plants show dark brown to black lesions on stems with dark streaking in the vascular tissue (See Photo). 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.
Black Rot (fungus – Ceratocystis fimbriata): Circular, almost black spots appear on sweet potato 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 sweet potato disease, black rot is difficult to find today.
Cold Damage (physiological): Pithy, dark discolorations will appear internally on sweet potatoes subjected to low temperatures. Temperatures do not have to reach the freezing point to cause damage. Sweet potato is a tropical plant and will suffer injury at temperatures below 55oF.
Cotton Root Rot – See Page on Cotton Root Rot.
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.
Growth Cracks (physiological): Scientists have not yet determined the conditions that cause cracking of sweet potatoes. In some varieties, it appears to be an inherited characteristic. Rotation reduces incidence of growth cracks.
Root Knot (nematode – Meloidogyne sp.): Root cracking is often associated with severe root knot infection (See Photo). 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 (See Photo). 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.
Russet Crack and Internal Cork (virus – Sweetpotato feathery mottle virus): The virus is transmitted by aphids and it is believed all sweet potatoes are infected. Symptoms, however, vary depending on strains of the virus and variety. Russet Crack is associated with Jersey-type sweet potatoes. Small, longitudinal cracks encircle parts of storage rots (See Photo). 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.
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 (See Photo). 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.
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 (See Photo). 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 sweet potato.
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.
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 sweet potatoes 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. Sweet potatoes 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 55oF to 60oF, and at 85% – 90% 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.
Sweet Potato Varieties and Their Reaction to Diseases
|Variety||Root Knot Nematode||Fusarium Wilt||Internal Cork||Sclerotial Blight||Soil Rot|
R = Resistant or Tolerant I = Intermediate S = Susceptible