Black rot is one of the most serious diseases of grapes in the eastern United States. Crop losses can range from 5 to 80 percent, depending on the amount of disease in the vineyard, the weather, and variety susceptibility. The fungus Guignardia bidwelli can infect all green parts of the vine. Most damaging is the effect on fruit. Later fruit infections can destroy many grapes, even the entire crop.
Infected leaves develop reddish-brown, circular spots (lesions) on the upper leaf surface. As the lesions mature, the center becomes brown and small, black, pimple-like fruiting bodies called pycnidia appear in the center. They are usually arranged in a loose ring just inside a dark border. Infected berries become dark brown and are covered with numerous black pycnidia on the surface. The berries eventually shrivel into hard, black mummies. Most serious fruit infections occur when the grape is pea sized or larger.
The black rot fungus overwinters in mummified fruit on the vineyard floor or in old fruit clusters that hang in the vines. The fungus can also overwinter within cane lesions. Spores of the fungus are produced within the diseased fruit and infect leaves, blossoms, and young fruit during spring rains. Fruit infections occur from mid-bloom until the berries begins to color. Mature leaves and ripe fruit are not susceptible. Very few fruit or leaves are infected after late July, and none are infected after the end of August. Black rot infections depend on the temperature and the length of time the leaves are wet. Infections occur if susceptible tissue remains wet for a sufficient length of time, depending on temperature (see side table).
Infected prunings and mummified berries should be removed, burned, and/or buried in the soil before new growth begins in the spring. In vineyards with susceptible varieties or where black rot was a problem the previous year, early season fungicide sprays should be timed to prevent the earliest infections. Should infections become numerous, protecting against fruit rot is very difficult later in the growing season. Planting resistant varieties is strongly suggested. Variety selections are presented in Table 6.3.