Several viruses can cause a mottling or so-called mosaic patterns to the leaves of sweet potatoes. These include internal cork, Russet crack, mild mottle and tobacco mosaic. Diseased leaves may be distorted and have a variegated pattern of dark green and lighter yellowish-green areas. Affected plants have shortened internodes and may have a substantial yield reduction.

Virus diseases are a major problem in sweet potato production, largely because these viruses are transmitted from infected roots to slips during propagation. The most important virus disease of sweet potato in the United States is the feathery mottle virus. Symptoms of this disease are quite variable and cultivar dependent. Symptoms have been listed under various disease names including russet-crack, internal cork, and chlorotic mottle. Initially, infected plants show a yellowing or vein clearing of the leaves. In some cases, small feathery yellow patterns develop along leaf veins. Affected plants commonly are stunted and appear unthrifty.

Internal cork appears as dry, corky, dark-colored clumps of tissue scattered through the flesh of affected sweet potatoes. Small, corky lesions may be found in the roots at digging time. Usually extensive corking does not occur until the roots are stored for several weeks. The symptoms are more serious when storage temperatures are high (above 60 F).

The corky spots may occur singly or in groups and are most readily detected by cutting the roots into thin slices. Severely diseased roots may develop so many corky lesions that they become practically useless for food. Russet crack symptoms are most noticeable on the surface of affected sweet potatoes and consist of areas containing numerous fine or coarser irregular cracks and browning of the surface. The cracked areas are irregular and they may be in bands or spots. The lesions may cover much or only a little of root's surface. Lesions may also develop on the stems to which the sweet potatoes are attached or on the underground parts of sprouts in the hotbed.

CONTROL

The virus persists in the roots of diseased plants. If these roots are used to produce slips for new plantings, then plants derived from the infected root will also be diseased. Therefore, it is extremely important to use virus-free seed stock. Purchase seed certified to be free of viruses. If seed is saved from last year's crop, use only those roots that show no symptoms of russet-crack or internal cork. Unfortunately, some infected roots remain symptomless, and there is no guarantee that these roots will be free of the virus. The virus also may be transmitted from plant-to-plant in the field by aphids; however, efforts to control the disease by decreasing aphid populations has had only limited success and is costly.

Web updated 9/01/06
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