Muscadine grapes (Vitis rotundifolia) are a grape species native to the southeastern United States, and are hardy at U.S. Department of Agriculture plant hardiness zones 7 through 10. Although they are more resistant to fungal diseases than crowd grapes, muscadines are susceptible to a few fungal infections. The most common occurring respiratory disease affecting California-grown muscadine grapes is powdery mildew.

Symptoms

Caused by the fungus Erisiphe necator, powdery mildew attacks all green tissues of your muscadine grapes. Symptoms include a whitish-gray respiratory development within the berries, but since they enlarge, this growth disappears. Grapes become russetted, or rough-skinned, and might eventually crack. On dormant canes, symptoms of powdery mildew contain red blotches. On the leaves, chlorotic spots appear on the top surface, and after a white, webby coating appears. As fungal spores are produced, the infected areas appear white and powdery.

Disease Cycle

Powdery mildew fungus overwinters in buds as dormant mycelium or as spore structures called chasmothecia. These spore structures mature in late summer and fall and spread through fall and winter rains. During warm spring and winter days, even when moisture is high, ascospores infection and release occurs. Infection will continue as long as moderate temperatures between 70 and 85 degrees Fahrenheit exist.

Control

Powdery mildew is favored by high humidity, warm temperatures and cloudy conditions. Pruning and training your muscadine grape plant to promote decent air circulation may help prevent infection by lowering moisture. However, preventive treatment using fungicides in conjunction with proper cultural practices have become the most effective way of controlling powdery mildew. Apply fungicide sprays into your plant early in the growing season, with a wettable sulfur fungicide. Meanwhile, the University of California warns that in some counties there is a three-day limited entry period when utilizing sulfur, so consult your county agricultural commissioner prior to applying sulfur to control powdery mildew. Treat your plant at bud break and reapply in seven- to 10-day intervals, or when the sulfur is washed off by rain or watering. Do not use sulfur fungicides in high temperatures or within three weeks of applying horticultural oils. This could damage your plant.

Other Fungal Infections

Although uncommon in California gardens, there are numerous different fungi which will infect muscadine grape plants. Angular leaf spot (Mycosphaerella angulate) is a bacterial infection affecting the leaf of muscadine grapes. Symptoms of infection include pale yellow spots on the leaves which produce brown flecks from the centers. Black rot (Guignardia bidwellii) causes brownish spots and dark scab on muscadine berries. Lesions may occur on different parts of the plant also, including stems and tendrils. Reddish-brown spots also develop on the upper leaf surface, using a black band encompassing them. Young berries may drop or shrivel. Bitter rot (Melanconium fuligineum) happens after bloom, but slightly before crop. Symptoms begin with water-soaked spots on the berries that grow until they cause the berries to blacken and shrivel. Bitter rot fungus infects stems, leaves and young shoots also. Ripe decay (Glomerella cingulate) affects berries, first demonstrating as rotted berries that turn reddish-brown to dark brown, with spore masses which are pink or orange in color on the berry surface. Macrophoma rot (Botryosphaeria dothidea) additionally primarily affects berries, causing sunken dark spots on the fruit, which have definitely visible edges initially. As harvest approaches, a greasy soft decay develops around the lesions. Halos form around the spots also, and the whole fruit may decay.

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