Diseases of Muscadine Grapes

E. S. Luttrell.

The muscadine grape (Vitis rotundifolia) is a species native to the southeastern United States. Several varieties of commercial importance have been developed from it. It is grown in most of the Cotton Belt in place of bunch grapes, which generally are unsuccessful in the region. The crop is used mainly for wine making and can be used for fresh juice, jellies, and frozen products. It is also sold as fresh fruit in local markets.

Black rot is its most common disease. It occurs on stems, leaves, flower clusters, and berries and is present throughout the growing season. It produces circular, reddish-brown spots on the leaves, black cankers on the stems and flower clusters, occasional blighting of the young stem tips and flower clusters, and brown or black scabs and cankers on the berries.

Infection of the foliage usually is rather heavy, but little defoliation occurs except on a few varieties of minor importance. The scabs and cankers on the berries may lower their quality but usually are responsible for losses of only a very small fraction of the crop. Some varieties are highly resistant or immune to berry infection.

Black rot is caused by the fungus Guignardia bidwellii, the same species that causes black rot of bunch grapes. However, the fungus on muscadine grapes represents a distinct physiologic form, which does not infect bunch grapes. The fungus produces conidia in tiny, spherical, black pycnidia, which are formed abundantly in the dead tissue of the spots and cankers during the growing season. Ascospores produced in perithecia on the over-wintered fallen leaves are chiefly responsible for the primary infection of the vegetative parts in early spring. The fungus also may survive the winter in the pycnidial stage in cankers on the stems and fallen berries.

The only other disease of any importance on the foliage is angular leaf spot, which is caused by the fungus Mycosphaerella angulata. It produces many small, angular, black spots, which are especially conspicuous on the lower surface of the leaf. Heavily infected leaves turn yellow and die. Infection is heavy on all varieties and considerable premature defoliation may result. In the Georgia Piedmont, however, angular leaf spot usually appears late in the season after the berries are mature. Probably it does not greatly affect the yield. In other sections it may develop earlier and be of greater importance.

The most serious diseases of muscadine grapes are the berry rots, of which bitter rot is the most important. The cause is Melanconium fuligineum, the same fungus that causes bitter rot of bunch grapes. Only the conidial stage of the fungus has been found. It may overwinter in that stage on berry mummies and dead pedicels. Bitter rot appears on only a few of the young green berries but spreads rapidly through the vineyard as the berries approach maturity. The berries are affected with a soft rot, which usually starts at the pedicel and spreads uniformly until the whole berry is involved. The surface soon becomes covered with a crust of black acervuli, from which the conidia may be rubbed in slimy, black masses. The entire berry finally shrivels to a hard, dry, black mummy. The mummies may cling to the vines, but usually the berries drop in the early stages of the rot. Many berries shrivel slightly and drop from the vines before they show any symptoms of rot. They drop singly or in small clusters clinging to shriveled pedicels. Apparently pedicel blight is merely another aspect of the bitter rot disease, the fungus spreading through the pedicels and branches of the peduncle from berry to berry or blighting the pedicels and causing the berries to drop before they are themselves invaded. Bitter rot usually is responsible for the drop of more than 60 percent of the berries that fall from the vines.

The only other berry rot of consequence, besides bitter rot and black rot, is macrophoma ripe rot. The causal fungus can be induced to develop a perfect stage, which belongs in Botryosphaeria ribis. But only the conidial stage, which would be placed in the form genus Macrophoma, is found on muscadine grapes. The fungus can overwinter in this stage on infected berries. The first symptoms are small, circular, slightly sunken, tan or brown spots on the surface of the berry. From the spots a brown rot extends irregularly over the berry, and the infected tissue becomes dotted with the black pycnidia of the fungus. The berry may shrivel but does not form a mummy.

Some degree of control of these diseases is doubtless had through such cultural practices as fertilization, pruning, and vineyard sanitation. Resistance to black rot and macrophoma ripe rot exists in some varieties. Although these berry rots are not important enough to make resistance to them a primary consideration in selecting a variety for planting, resistance could be incorporated in new, higher yielding varieties.

No spray schedule has been recommended specifically for muscadine grapes.

E. S. LUTTRELL is plant pathologist at the Georgia Agricultural Experiment Station, a position he has held since 1942, except for 2 years when he was assistant professor of botany in the University of Missouri.