Back to Extension publication listingFloyd F. Hendrix, Research/Extension Plant Pathologist
Disease control is a must for Georgia bunch grape producers. In 1968, growers lost one to five tons of grapes per acre to rot. The cost of disease control is only a small fraction of this loss. Controlling diseases on grapes is not difficult, provided you follow strict disease control practices. These practices are pruning, cultivating and spraying. If you are not prepared to carry out all three of these practices, you will not be a successful grape grower.
Diseases affect European grapes (Vitis vinifera) much more severely than they do American bunch grapes (Vitus labrusca). Black rot, anthracnose and powdery mildew are of major importance on both types of grapes. Downy mildew, Phomopsis leaf and cane blight and Botrytis blight cause devastating diseases on European grapes but only minor problems on American bunch grapes which usually require no special sprays.
Pruning: Severe annual pruning is a very important practice for controlling anthracnose and black rot. Since these two diseases overwinter in old canes, it is necessary to prune out and burn all excess growth. The only part of the vine kept year after year is the trunk.
Only strong, healthy canes of the previous year's growth are selected to produce the following season's crop. The selected canes should be as free of disease spots as possible. After fruiting canes have been selected, remove excess growth, dried berries and leaves. The remaining vine should contain only the permanent trunk, one-year-old fruiting canes and short spurs to produce new canes. The amount of diseased material left on and around vines after pruning influences the effectiveness of the spray program.
Cultivating: The fungus which causes black rot can also come from old berries and leaves on the ground. Cultivation of the soil just before new growth begins in the spring covers old berries and greatly reduces black rot infection. Timely cultivation or the use of herbicides throughout the spring and summer controls weeds which might prevent good air circulation and hinder spraying operation.
Spraying: You must apply several properly timed fungicide sprays to control grape diseases. Your success as a grape grower will depend on your understanding of spray equipment and the application of spray material. Refer to Georgia Farm Chemical Handbook for specific fungicide recommendations.
Black rot is the most destructive grape disease in Georgia. Therefore, the entire cultural program should be pointed toward its control. Other diseases will be less troublesome when recommended practices for black rot control are carried out. Understanding this disease will enable you to produce greater yields of high-quality grapes.
Source of infection: The fungus overwinters in infected canes and grapes, spreading to new growth early in the spring as green tissue appears and suitable weather conditions develop.
After the disease becomes established in new shoots, leaves, tendrils and blooms, the fungus reproduces in great quantity and spreads rapidly in recurring waves during each rain. Grapes become infected by the time they are fully grown, dry and shatter to the ground. There they remain as a source of disease the following year.
Symptoms: The black rot fungus attacks all parts of the grape vine. Leaf infection appears as tiny, reddish-brown spots on the upper surface in early June. The lesions enlarge to one-forth inch or more in diameter and become brown with black borders. A ring of black fungus bodies develops near the outer edge of the brown area. Lesions on stems and tendrils are longer and blacker than those on leaves. Stem lesions are narrow, sunken and often split lengthwise on the vine.
Infections begin to appear on the fruit when the grapes are about half grown. At first, a small, white spot forms. It enlarges rapidly until the entire grape is rotten. Affected grapes soon turn black, shrivel and dry up. Minute, black fungus fruiting bodies develop on the surface of the dried fruit.
Anthracnose may cause extensive losses in poorly kept Georgia vineyards, but it can be controlled economically.
Source of infection: Anthracnose overwinters on infected canes. It spreads to all new growth during wet periods in early spring.
Symptoms: The appearance of anthracnose on fruit, stems and leaves is not readily confused with black rot and other diseases. Fruit infections appear as a "bird's eye" effect, having light-gray centers and reddish-brown borders. Stem lesions are similar in color, sunken, with slightly raised borders. Severely infected leaves become distorted and curl down from the margins. Individual spots are gray with a dark border. Later, the center of the lesion drops out, giving a ragged effect.
Plasmopara viticola, the cause of downy mildew, attacks all green parts of the vine. Initially, lesions are yellowish and oily, and become angular, yellow to reddish brown spots. Infected shoots thicken and curl, then turn brown and die. Young berries become gray when infected. Rachis infection can spread into older berries causing a brown rot. All infected parts, except older fruit, are covered with white fungal growth during moist weather. The fungus overwinters primarily in infected leaves on the ground. It may survive as mycelium in buds during mild winters. Pre bloom sprays are necessary for control.
This disease is especially destructive in climates like the one in Georgia. Leaves have small chlorotic, irregular to circular spots with dark centers. Spots may also occur on veins and petioles, and leaf margins may be turned under. Dead areas may drop out leaving a "shot-hole" appearance. Infected rachis, stems and petioles have dark brown to black streaks or blotches. Cluster stems may become brittle and break. Infected fruit become dark brown and brittle. Proper pruning is essential for the control of this disease. Early sprays are particularly important.
Powdery mildew can infect all green parts of vines. Symptoms are a white powdery growth on infected parts. Berries infected before they are fully grown split and either dry up or rot. Infection of fully grown fruit results in a blotchy appearance or a netlike pattern of scar tissue. Infected fruit may cause off flavors in wine. The fungus overwinters as black fruiting bodies on the vines and in dormant buds. Sprays starting at petal fall are needed for control of this disease.
Botrytis infects primarily at bloom, killing the flowers. From the blooms it infects the rachis, girdling it and killing the cluster of fruit from that point outward. Fruit may also be infected during ripening. The disease progresses through the whole cluster. White berries turn brown, and dark berries become reddish. The fungus overwinters on hard black bodies, called sclerotia, on the canes or soil surface. Bloom sprays are essential for control of this disease.
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