Black rot, caused by the fungus Guignardia bidwellii, can cause heavy losses to infected grapes, especially during warm, humid weather. Black rot starts as tiny yellow spots that grow to about 1/4 inch on the leaves. 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 the 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 leaf spots. Black rot is a common Iowa grape disease.
The primary winter survival mode for the fungus is in mummified berries. However, other infected plant material can be a safe haven for the fungus.
Management
Management of these grape diseases consists of the following:
- Prune and destroy diseased plant material while plants are dormant to reduce the potential for disease the following year. Be sure to eliminate black rot mummies.
- Remove wild grapes around the area because these grapes can harbor disease organisms.
- Avoid planting grape plants that are susceptible to anthracnose.
- Promote canopy drying by pruning, training, row orientation, and proper spacing.
If your planting has had problems with black rot in the past, a delayed dormant spray (just before the buds break) of liquid lime sulfur will help to reduce the threat for the upcoming season. In addition, you may wish to consider a program of preventive fungicide sprays (mancozeb or captan) at 2-week intervals from bud break until the fruit begin to turn color. Prune out damaged shoots and clusters and remove the prunings from the vineyard, and rake the ground under the plants in fall in order to remove all fallen berries.
Photo credits: 1: University of Georgia Plant Pathology, University of Georgia, Bugwood.org; 2: University of Georgia Plant Pathology, University of Georgia, Bugwood.org; 3: M. Clerjeau, INRA, Centre de Recherches de Bordeaux, Bugwood.org
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