The berries on my grapevines have shriveled up and turned bluish black. What happened to my grapes?
Black rot is probably responsible for the damage to the grapes. Black rot is caused by the fungus Guignardia bidwellii. This fungal disease is common in home and commercial vineyards in Iowa, especially in warm, humid summers.
On grape foliage, black rot starts as tiny yellow spots that grow to about one-quarter inch in diameter. The centers of the spots turn rusty red. Tiny black dots usually form in the center of the spots. These dots contain thousands of spores that can produce new infections. Symptoms on fruit become apparent when the berries are about half grown. Infected berries can rot in a few days. They shrivel and become hard, black mummies, which contain the same tiny spore containing structures as the leaf spots.
The black rot fungus survives the winter primarily in mummified fruit on the vine or on the ground. However, it also overwinters on diseased areas on the vine.
Sanitation is an important step in controlling black rot. Rake up all fallen mummies on the ground. Also remove any mummies that are still hanging onto the vines. The mummies should be removed from the area and destroyed. A preventive fungicide spray program, beginning shortly after bud break and continuing until the fruit begin to develop color, should help to suppress black rot in plantings where the disease has been a problem in previous years.