Healthy grapes have a natural, slightly powdery, bloom.

How to Control Powdery Mildew on Grapes

by Jenny Green

Plucking grapes fresh from the vine to pop in your toddler's mouth gives her an introduction to healthy eating, a small lesson on where food comes from and a vitamin boost, but isn't possible if your grapes are covered in powdery white mold. Grapes are members of the genus Vitis, and wine and dessert varieties range in hardiness between U.S. Department of Agriculture plant hardiness zones 4a and 9b. Home gardeners can control powdery mildew on grapes manually and through fungicide applications.

1. Manual Control

Manual control of powdery mildew helps prevent the disease from overwintering in bark crevices and the spread of current infections. Powdery mildew infects leaves, shoots, bark and developing fruit on grapevines, releasing spores in spring during warm, humid conditions. Removing diseased leaves and shoots as they appear and removing dead plant material at the end of the season helps prevent powdery mildew spreading and overwintering on plants. Spring pruning removes more potential sources of infection. Prune grapevines once hard frosts are over, cutting away all canes except one or two young, unfruited canes 8 to 10 buds long on either side of the grape trunk. Tie these to supporting wires.

2. Fungicides

Sulfur, oils and biological controls are available to the home gardener for powdery mildew control. Apply sulfur before symptoms appear, using a product with added surfactant to help the sulfur stick to plant surfaces. You can also apply horticultural oils, neem oil and jojoba oil as a protectant or to eradicate existing infections. Sulfur and oils should never be applied together or within two weeks of each other, and neither should be used once temperatures exceed 90 degrees Fahrenheit. Biological powdery mildew control contains the bacteria Bacillus subtilis, which helps prevent powdery mildew infections spreading. Follow manufacturers' instructions for all products, and keep fungicides well out of the reach of children.

3. Application

Fungicide applications to control powdery mildew should begin before flowering and continue while conditions for infection are favorable. Spray a protectant when flower buds are full and about to burst, wetting all plant surfaces but not saturating them, which causes the fungicide to drip off. Repeat the spraying every 10 days if last season's disease was severe. Intervals between spraying can be lengthened if symptoms don't appear and temperatures exceed 90 degrees. If powdery mildew takes hold, continue spraying until leaf fall to help reduce next season's infection rates. Spray with ready-to-use fungicides or dilute according to manufacturers' instructions.

4. Tips

Growing grape varieties resistant to powdery mildew and providing suitable growing conditions can help eradicate powdery mildew infections in home garden grapevines. Some grape cultivars, such as Traminette, Canadice, Cayuga White, Norton, Ives, Marquis, Mars and Steuben are resistant to powdery mildew. Growing resistant varieties, well-spaced apart in open, sunny areas should reduce infection rates. If leaf cover is thick, expose developing grapes and improve air circulation by removing leaves at the bases of the stems.

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