Black Rot (Ceratocystis fimbriata)
Small, circular, slightly sunken, dark brown spots are the initial symptoms of black rot. Spots enlarge and appear greenish black to black when wet and grayish black when dry. Within the spots are small, black fungal structures (perithecia) with long necks which appear to the naked eye as dark bristles. The rot usually remains firm and shallow. If secondary fungi or bacteria invade the tissue however, the flesh beneath the spot turns black, and this blackened area may extend to the center of the root. Tissue near the discolored area may have a bitter taste. Eventually, the entire root may rot. Roots may appear healthy at harvest but rot in storage, during transit, or in the market.
Control black rot with crop rotation, since most crops are unaffected by the disease. Disinfect seedbeds if a clean site is unavailable. Propagate plants from healthy stem cuttings. Cure roots immediately after harvest. (Cure roots at 85 to 95 ° F and 85 to 90 percent relative humidity for 5 to 10 days). Do not wash and package roots showing symptoms of black rot. Decontaminate equipment that comes into contact with an infected crop.
Rhizopus Soft Rot (Rhizopus stolonifer)
Infection and decay commonly occur at one or both ends of the root, although infection occasionally begins elsewhere. Rotting may be inhibited under dry conditions, but under humid conditions the affected sweet potatoes become soft and watery, and the entire root rots within a few days. If the humidity is high, the sweet potatoes become heavily "whiskered" with a grayish black fungal growth. This feature distinguishes Rhizopus soft rot from other storage rots. The color of the root is not significantly altered, but an odor is produced that attracts fruit flies to the area. Infection is especially likely if the relative humidity is between 75 and 85 percent during storage or transport. Also, the longer roots are stored, the more susceptible they become. Chilling and heat damage also predispose sweet potatoes to infection. Soft rot is very destructive when sweet potatoes are washed, packed, or shipped to market during cold weather.
Carefully handle sweet potatoes during harvest to prevent unnecessary wounding. This is the most important control method for soft rot. Properly cure roots immediately after harvest. Store roots at 55 to 60 degrees F. Avoid handling stored roots because handling can create new wounds. Recuring is one possible solution to this problem. Apply a recommended fungicide after harvest. Do not allow sweet potatoes to be exposed to sunlight for extended periods (to prevent heat damage) or to be chilled in the field.
Bacterial Soft Rot (Erwinia chrysanthemi)
Roots are affected in the field, or more commonly in storage, by a soft rot that turns diseased tissue light brown and watery . Lesions on storage roots often have a dark brown margin. Some storage roots appear healthy from the outside but are decayed internally. Infected roots show black streaks in the vascular tissue and eventually undergo a soft, moist decay. Mother roots often decay in plant beds. In the field, brown to black, water-soaked lesions appear on stems and petioles. Eventually, the stem may become watery and collapse, causing the ends of vines to wilt. Usually, one or two vines may collapse, but occasionally the entire plant dies.
Carefully handle sweet potatoes during all stages of production. This is the most important control method for bacterial soft rot. Select mother roots from fields free of the disease. Cull roots infected during storage. Use vines cut above the soil surface for transplanting. Use a handling system that does not involve immersion of sweet potatoes in water.
Charcoal Rot (Macrophomina phaseoli)
In the field, brown to black, water-soaked lesions appear on stems and petioles. Eventually, the stem may become watery and collapse, causing the ends of vines to wilt. Usually, one or two vines may collapse, but occasionally the entire plant dies. Charcoal rot, caused by the fungus, can cause losses of sweet potatoes in storage, but serious losses seldom occur. The disease is sometimes confused with black rot and Java black rot. Symptoms in storage begin as a reddish brown to brown, firm, moist rot, initially restricted to the area just beneath the sweet potato skin. As the decay progresses, the pathogen moves toward the center of the sweet potato, causing further rot. Two distinct zones become apparent within the infected tissue. The leading edge continues as a reddish brown decay, and a zone of black develops behind the zone of active decay. Although the lesions are sometimes restricted, charcoal rot usually consumes the entire root, which eventually dries, becoming hard and mummified.
Properly cure sweet potatoes immediately after harvest to reduce the incidence of charcoal rot.