Don't let fungi sour your grapes

Photo of a grape with botrytis.
Grape with botrytis. (Photo by Joseph Smilanick, USDA.)
Last Updated: 
August 10, 2012

CORVALLIS, Ore. – After carefully nurturing homegrown grape vines, don't let fungi sour your grapes.

So be on the lookout for powdery mildew and bunch rot during the summer, advises Ross Penhallegon, a horticulturist with the Oregon State University Extension Service.

Powdery mildew attacks all parts of the vine, leaves, stems and fruit. It's usually worse on vines in areas where air circulation is poor or in places that remain damp during the early warm part of the day. Water on the foliage may also cause mildew.

In its early stages, powdery mildew appears as whitish or grayish patches on the leaves. Affected fruit may also appear gray or white at first. Later, the skin of the grapes becomes brown and roughly mottled. Infected fruit may shrivel, fail to mature and, often around the first of September, crack.

To control powdery mildew, Penhallegon recommends pruning the grape plant to allow air to circulate and sunlight to penetrate through the vine. Then, using a sprayer, apply wettable sulfur to the vine and leaves. Several sprays – both conventional and organic – will work. Follow directions on the label. Spray all foliage and fruit, covering tops and bottoms of all plants. Apply once a week until the fruit begins to change color.

Bunch rot (Botrytis) will sometimes show up when it rains at harvest time or when fruit is hit with overhead sprinklers. Symptoms include rotted fruit with tufts of gray fungi growing on the surface of the grapes. For control, gardeners can use captan fungicide, Stylet-Oil or products containing copper. It also helps to cut out badly infected bunches of grapes and to prune to provide better air ventilation through the vines.

The Pacific Northwest Plant Disease Management Handbook offers more information on powdery mildew and botrytis.

To learn more about growing table grapes, check out the publication Growing Table Grapes from OSU Extension Service.

Author: Tiffany Woods