Do Your Grapes Suffer from Black Rot?

Grapes are one of the most ancient crops known, and I have noticed that many homeowners in this part of Pennsylvania have a grape vine or two in their backyard.

When a homeowner calls the Extension office and says they have a problem with their grape vine, and that the grapes turn black and hard before they even ripen, I’m pretty sure that black rot is the culprit. Black rot is caused by the fungus Guignardia bidwelli. This fungus attacks all parts of the grape vine, but it is the fruit infection phase that makes people pick up the phone, call Penn State Extension, and ask for help.

The fungus overwinters in diseased fruit and canes either on the ground under the vine, or still connected to the vine. Fungal spores are produced in this diseased tissue, and begin to infect leaves, blossoms, and young fruit during spring rains. Fruit infection occurs from mid-bloom until the berries begin to color. Infected berries become dark brown and are covered with numerous black spots on the surface. The berries eventually shrivel into hard, black mummies.

So what can be done to control black rot? First, practice good field sanitation. That means removing all prunings and infected berries from the area before the new growth begins in the spring. If black rot has been a problem in the past, it is recommended to apply a labeled fungicide as soon as the grapes begin to flower, and to continue applications until berries begin to mature. Finally, prune and trail grapes properly so as to maximize air movement through the plant canopy, which will help slow the spread of this disease.

For more information on this and other grape diseases, see the Penn State Fruit Production for the Home Gardener publication.