University of Vermont Extension
Department of Plant and Soil Science
Summer News
Article
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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