University of Vermont Extension 
Department of Plant and Soil Science

Summer News Article
line

WATCH FOR THESE GRAPE PROBLEMS

 
Dr. Leonard Perry, Extension Professor
University of Vermont


Mildews and black rots are some diseases to watch for on grape plants, while main pests include the grape berry moth, Japanese beetles, and rose chafers. 
           
Good management such as selection of disease resistant cultivars (cultivated varieties), proper and sufficient pruning in spring to allow air circulation, and keeping leaves and fallen fruit raked up in fall, go a long way to preventing diseases.  If your vines, however, don’t escape insects or disease, you may be able to pick them off if just a few leaves affected or insects caught early, or these vigorous plants can tolerate low levels.  If you do use sprays, make sure they are labeled for the problem being treated, that you correctly identified the problem (trained professionals at nurseries and university plant clinics may help), and that you follow all label directions and precautions.
           
If you suddenly see stunted and deformed growth, this symptom is often not of a disease but rather a reaction to herbicides applied nearby.  This is quite common in grapes grown in or near lawns to which weed and feed products have been applied.  Grapes are quite sensitive to herbicides, so don’t use them anywhere near your vines.
           
Black rot is a fungal disease which covers the leaves with brown spots and black pimples, and turns the fruit black, rotten, and shriveled.  It can occur any time during the season during warm and wet conditions.  Good sanitation by cleaning up old fruit and leaves will help greatly by preventing it from overwintering.
 
There are two different mildew diseases to watch for on grape vines.  Downy mildew covers leaves, new shoots, and fruit with a gray down, and eventually rots fruit.  Powdery mildew shows up as a white, velvety substance covering leaves, twigs, and fruits.  This is similar, but not the same organism, as causes mildew on flowers such as zinnias and lilacs, or vegetables such as squash.   If using sulfur-based sprays on these mildews, keep in mind that some American grape cultivars such as Concord, Chambourcin, Foch, and Leon Millot are sensitive and can be damaged by them.

Another disease you may see when conditions are moderately warm and wet are fruit rots caused by gray mold (botrytis).   These are common on cultivars with dense fruit clusters.  Early in the season, buds and young fruit turn brown.  During the season you may see large, reddish-brown dead areas on leaf edges. Fruits turn color and rot.  Remove infected leaves, thin clusters, prune to increase air circulation, and hope for drier weather.
           
Grape berry moths are the main source of wormy grapes, and perhaps the main insect pest in many areas. Their larvae feed on flowers and young tender growth in spring, then enter young fruit where they eat the pulp. Look for the webbing in which they often encase themselves, or for reddish spots on berries.  You can control them by picking off infected fruit if not many, by removing leaf litter under plants in fall, or by insecticides early in the season.
           
Japanese beetles particularly are attracted to grapes and many members of the rose family, including roses and brambles.  Simply knock them off into a pail of soapy water. Traps for these are widely available and commonly used.  Since these are quite attractive to the beetles, which feed on your plants en route, place traps as far away from your grapes as possible.  Milky spore is a biological control that works on the beetle grubs in warmer climates (zones 6 and warmer).
           
Rose chafers eat blossoms, buds, and newly formed fruit early in the season. The straw-colored beetles, about a half-inch long, also skeletonize leaves in June and July.  Usually they are most troublesome on vines grown on sandy soil. If there are just a few, knock them off into a jar of soapy water. Check frequently, as more will likely fly in.
           
While these are the main pests to watch for, others that might show up in some areas are cane girdlers, grape flea beetles, and common pests such as aphids, leaf hoppers, leaf rollers, and mites.  Racoons, skunks, and opossums also like grapes, and may beat you to the harvest. The best control for these is a low electric fence, 6 inches off the ground.
 
In regions where birds steal the fruits, you may need bird netting. Suspend the netting above the plants, as laid right on the vines the birds can reach through to the fruit.  Or grow Concord grapes, which they often don’t bother.  Other bird repellents such as aluminum pie plates and reflective balloons blowing in the wind are only marginally effective; placing paper bags over each cluster works for people with time or just a few fruit.

When harvesting, watch out for yellowjackets that can damage the picker, as well as the fruit.  They are attracted in late summer and fall to the sweet sugars of overripe fruit.  Keeping fruit picked, or fallen grapes raked up, will help keep these away.
           
Grape vines in home gardens often avoid the problems that commercial growers with large acreages have to control.  While you should scout your vines weekly, if not more often, for these pests and diseases, you may see few if any.  More information and photos, on these and other problems, can be found online from the Cornell University IPM program (www.nysipm.cornell.edu/factsheets/grapes).

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