POWDERY MILDEW
Description
Symptoms
Disease Cycle
Control

Description:
Powdery mildew is one of the most common grape diseases worldwide. All varieties of Vitis vinifera, French-American hybrid, and V. labrusca grown in the Southeast are susceptible (Table 1). Varieties of muscadine grapes (V. rotundifolia) vary in susceptibility from almost immune to susceptible. Severe infections can reduce vine growth and yield and predispose fruit to rot fungi (Table 2).

Symptoms:
The disease is named for the ash-gray to white growth of the fungus on the surface of infected leaves and fruit. Infections on leaves first appear as small yellow green blotches, about ? inch in diameter, on the upper leaf surface. As lesions enlarge they become covered with the diagnostic white mycelial growth (Fig.1). On some varieties, veinlets on the lower leaf surface turn brown beneath the lesions. Young heavily infected leaves may become distorted. Lesions tend to ?disappear? during hot summer weather, often leaving darkened areas on the leaf where the infections were present. Infections of fruit and cluster stems are characterized by ashy gray to white growth on the surface (Fig.2). Fruit infections later appear as web-like russet on the surface. Heavily infected fruit often split and crack as they mature (Fig.3).

Disease cycle:
The causal fungus, Uncinula necator, overwinters as hyphae in dormant buds or on canes as sexual fruiting structures known as cleistothecia. Leaves emerging from infected buds are covered with whitish mycelium and conidia (spores) that blow to emerging leaves and fruit clusters, initiating infections. Primary infections can also occur from ascospores produced in cleistothecia. These infections are most common on the lower leaf surface of leaves growing near the bark of the canes where the cleistothecia have overwintered. Temperatures of 20 to 27 ?C (~68 to 81 ?F) are optimum for infection, though infections can occur from 6 to 32 ?C (~43 to 90 ?F). At optimum temperatures, lesions can develop in as few as 5 to 7 days. Unlike most diseases, rainfall inhibits disease development. Periods with high humidity (85% is optimum) without free moisture on the leaf surface favor disease development. Numerous secondary infection cycles can occur during the growing season. Fruit are susceptible from just before bloom until about one month after bloom. Inconspicuous ?diffuse? infections on berries can increase the severity of berry rots at harvest. New leaves are susceptible through the growing season, though the disease usually becomes less active during the hot summer months, and becomes active again in the late summer and fall once temperatures cool.

Control:
Cultural - Cultural practices are important in reducing disease severity. Select planting sites with good air circulation and good sun exposure. Training and pruning practices that open the vine canopy to allow air movement can help reduce disease severity. Varieties vary in their susceptibility (Table). Some varieties of V. rotundifolia are almost immune.

Chemical - On V. vinifera, French American hybrids, and varieties of V. labrusca, sprays for powdery mildew control should begin at 3 to 10 inches of shoot growth (begin at 3-5 inches of shoot growth where powdery mildew has been a problem in the past) and continue on a regular schedule until 4 weeks after bloom. Sprays beyond this time may not be needed but vines should be scouted on a regular basis for new outbreaks. The disease often becomes a problem after harvest, and vines should be scouted regularly to determine if sprays are needed at this time. Failure to control postharvest outbreaks of powdery mildew may result in early defoliation, predisposing the vines to winter injury. On varieties of V. rotundifolia susceptible to powdery mildew, begin fungicide sprays at first bloom and continue for 4 to 6 weeks until fruit sizing. Rotate fungicides and use the full labeled rate to avoid the development of resistance. Refer to the Winegrape Spray Program http://ipm.ncsu.edu/agchem/chptr7/706.pdf and Muscadine Spray Program http://ipm.ncsu.edu/agchem/chptr7/707.pdf in the North Carolina Agricultural Chemicals Manual for fungicide recommendations for powdery mildew control.