Black rot, caused by the fungus Guignardia bidwellii, is one of the most serious diseases of grapes in the eastern United States. Crop losses can be devastating, ranging from 5 to 80 percent depending on the amount of disease in the vineyard, weather, and cultivar susceptibility. The fungus can infect all green parts of the vine including, fruit, shoots, leaves, and tendrils. Most damaging is the effect on fruit.

Symptoms

Reddish-brown, usually circular leaf spots first appear on the upper leaf surface. Soon after, the center of the spot becomes tan to light brown. Small, black, pimplelike bodies (pycnidia) appear in the center of the spot, usually arranged in a loose ring just inside the dark brown edges of the spot. On petioles, black, elongated lesions may induce wilting of leaves. Infected berries initially turn brown in color, and may remain as a cluster on the vine or fall to the ground as individual berries leaving only the rachis. The final stage is a dark purple to black, wrinkled mummy. On shoots, lesions may range in size from less than a 1/16 to 3/4 of an inch across, appearing as circular to oval-shaped, dark-brown to black, sunken cankers; larger lesions appear lighter in color. Pycnidia may be clumped in the center of the lesion; these lesions remain on the shoots after they have "hardened off" and can survive over winter. Large shoot lesions may render the shoots susceptible to breakage by wind.

Disease Cycle

The fungus overwinters primarily as mummified berries on the soil or in old berry clusters that hang in the vines, but can also over winter on cane lesions. Spores of the fungus are produced within the diseased fruit and canes and released during spring rains, infecting leaves, blossoms, succulent shoots, and young fruit. Spores within cane lesions are available for infection in early spring at bud break; however, the vast majority of spores (those within berry mummies) first become available about 2 to 3 weeks after bud break, then reach peak levels from about 1 to 2 weeks before bloom until about 1 to 2 weeks after, depending on the year. Fruit infections occur from midbloom until the berries begin to change color (veraison). Most serious fruit infections occur when the grape is pea sized or larger. 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 temperature and the length of time susceptible tissues remain wet.

Leaf Lesion	Mummified Berry	Shoot Lesions

Disease Management

Infected prunings and mummified berries should be removed and disked into the soil before new growth begins to reduce disease pressure. In vineyards with susceptible cultivars or where black rot was a problem the previous year, early season fungicide sprays should be timed to prevent the earliest infections. Cultural practices that open the canopy are beneficial because they increase air circulation and improve spray coverage. Should infections become numerous, it is very difficult to protect against fruit rot later in the growing season. It is strongly suggested to plant resistant cultivars.

Removal of mummified clusters during pruning significantly reduces disease pressure for the coming season; spring cultivation to bury mummies also can contribute to a reduction of inoculum. Black rot management using fungicides can be achieved with either a protective or postinfection program.

Protectant fungicide applications usually begin near the 3- to 5-inch shoot growth (sometimes a bit earlier or later, depending on weather and disease levels the previous year), and continue at regular intervals of 10 to 14 days until berries reach 1/4-inch diameter.

A postinfection program requires accurate monitoring of weather conditions. Postinfection sprays are applied after an infection period has occurred. Good management early in the season often allows black rot management programs to end once fruit reach 1/4 inch in diameter.