My grapes are beginning to turn brown and shrivel. What is the cause of the problem and the prevention?
Black rot, the most widespread disease of grapes, often causes grapes to turn brown and shrivel. Most prevalent in warm humid weather, this disease can cause a complete crop loss if proper precautions are not taken. Many grape cultivars have some resistance but still require a few applications of recommended fungicide sprays during wet periods. Cultivars that have large fruits or mature early in the season are the most susceptible.
Symptoms of black rot may first appear on the leaves as small light tan spots with circular centers and darker tan margins. Fruit symptoms do not appear until the berries are half grown. Then berries turn brown and rapidly shrivel, becoming small, black, and hard.
Once black rot is observed, very little can be done that season. Three suggestions to help you reduce the incident of black rot in the future:
1. Practice good sanitation. When black rot appears, begin sanitation cleanup at once, writing off the current crop. Collect and destroy all fruit because any that is left behind will be a source of infection next year. In late winter, just before buds begin to break open, remove all of the previous season's fruit-bearing stems, which likely still harbor disease, and begin a spray program.
2. Make sure you provide good air movement around the plant. Select a sunny open area and use proper row orientation. Perform yearly pruning.
3. Use fungicides as soon as shoots begin to emerge from the vine. During wet weather, applications should not be more than seven to 10 days apart. Contact your local Extension office for recommended products.