Muscadine Fungus

"It’s not the heat; it’s the humidity." During the summer season, this lament is not restricted to the human species, but it’s also descriptive of a climatic challenge to growing grapes. Muscadine grapes (Vitis rotundifolia) are adapted to growing in U.S. Department of Agriculture hardiness zones 7 and warmer, particularly where summers are long, hot and humid. These same environmental conditions also encourage fungus diseases that reduce yields and compromise plant health.

Muscadines typically withstand diseases better than other grape species. Although they are virtually immune to Pierce’s disease, phylloxera and nematodes, which pose serious threats to other grapes, they are susceptible to other diseases. When they do succumb to an illness, a fungus is generally the culprit. In their native habitat of the southeastern United States where higher humidity favors fungal growth, muscadines are susceptible to more diseases than in drier Mediterranean climates. Leaf diseases reduce grape yields, but fruit rots are typically more pervasive and damaging.

Although muscadines do not succumb to downy mildew (Plasmopara viticola), which infects other grape species, they are at risk of infection from powdery mildew (Uncinula necator). This fungus attacks muscadine fruit, and it is recognized in its early stages by the white cottony growth that covers grapes. As the fruit matures, the white growth disappears to reveal rough-skinned grapes that may split. Grapes may drop from vines prematurely while those that remain attached may fail to reach mature size.

Fruit rots primarily target muscadine fruit, but they may also infect stems and leaves. Although bitter rot (Greeneria uvicola, syn. Melanconium fuligineum) attacks young fruit shortly after the flowers fall, symptoms don’t generally occur until just before harvest. Grapes turn black and fruit walls may erupt with spore structures. Macrophoma rot (Botryosphaeria dothidea) causes black spots to form on fruit before the entire grape rots. Black rot (Guignardia bidwellii f. muscadinii) causes black scabs on fruit, spots on leaves and lesions on stems.

Leaf spots may seem innocuous and only cosmetically detrimental, but they can be caused by diseases that compromise grape quality and harvest yield. Angular leaf spot (Mycosphaerella angulata) is a primary fungus disease of muscadines. Initial symptoms appear as yellow stippled spots on leaves. These spots grow until irregular brown patches appear inside. As the disease progresses, leaves fall from vines. In severe cases, vines may become denuded. Severe defoliation reduces yields and diminishes grape quality.

Selecting disease-resistant cultivars is the first step toward preventing fungus problems. Dark-skinned muscadine cultivars, such as "Noble" and "Pride," generally have higher resistance to fungus. Because fungus overwinters in dropped leaves and mummified fruit, scrupulous sanitation is essential to preventing diseases from re-infecting vines each year. The Mississippi State University Extension Service recommends a spray regimen for preventing fungus diseases. Critical application times are just before and just after bloom, with repeat applications every two weeks. Different products have varied recommendations for the length of time you should discontinue their use before harvest, from zero days (Captan) to 66 days (Mancozeb).