The grape clusters on the vines have spots on them. What caused it and what, if anything can be done about it? What causes the grapes on my vine to turn dark and shrivel up like raisins?

Grapes March 16, 2012|Print

Black rot, caused by the fungus Guignardia bidwellii, consistently destroys grapes in the Midwest and Eastern USA, especially during wet seasons. During susceptible stages of growth (especially newly expanding plant parts) spores of the fungus are splashed to susceptible tissue and infect when free moisture is present. The disease slows down in mid-summer as leaves, stems, and fruit mature. Fruit infection begins as a dark rotted spot, but soon spreads to the entire berry and to other fruits in the cluster. Berries turn brown and rapidly shrivel, becoming small, black, and hard. Black rot disease is also observed on grape leaves as circular, tan spots with dark margins. Black rot management begins in winter with sanitation. Shriveled fruit left over from the summer must be removed from the vines and picked up off the ground and destroyed. Because of black rot, many grapes are difficult to grow without the benefit of regular fungicide applications from bud break until just before 4-6 weeks after bloom. Cultural practices that promote fast-drying of the plants following rain or dew will aid in black rot control. These practices include proper vine spacing, selection of a well-drained planting site exposed to full sun, proper pruning, and removal of tall weeds. Reduce primary (spring) infection by burning or otherwise destroying material removed during winter pruning and diseased and overwintering berries and leaves. Varieties of grapes differ in susceptibility to black rot, but most wine and table grape varieties are quite susceptible. Apply fungicide sprays during the critical early-season period when the first infections are taking place. If not controlled early, the fungus may attack the young tissues and build up a reservoir of spores capable of infecting the fruit later in the season. Spray the vines every 7 to 14 days, beginning when the longest shoots are 3 to 6 inches long and continuing until 4-6 weeks after bloom. Although black rot is most common, other grape diseases such as bitter rot, ripe rot, and anthracnose can also cause fruits to turn black and shrivel up. Diagnosing these fruit diseases might require the services of a professional diagnostic laboratory. 1. Practice good sanitation. When black rot appears, begin cleanup of diseased material at once. Collect and destroy all fruit because any that are 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 spores of the black rot pathogen. 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 7 to 10 days apart. Contact your local Extension office for current fungicide recommendations.

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