Anthracnose and black rot are two common Iowa grape diseases. They both like the same environmental conditions, so often one plant will have both diseases, which can lead to confusion.
The anthracnose fungus, Elsinoe ampelina, overwinters in infected plant material. In the spring when the temperature is at least 36 F, spores are spread by wind and splashing rain and can infect all aboveground parts of the plant. However, warmer conditions (up to 90 F) promote more severe infections.
Small, round, reddish spots begin to form on leaves as symptoms begin. On stems and fruit, spots develop into sunken, gray areas with dark reddish to purple edges. Spots can run together and kill entire shoots or berries. As spots develop on leaves, they form gray centers with dark edges that are typically angular rather than circular.
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 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.
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 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.
These practices can help improve air circulation.
See the Midwest Commercial Small Fruit and Grape Spray Guide for fungicide scheduling.
For more information see the references below.
Small Fruits, Insect and Disease management for Backyard Fruit Growers in the Midwest IDEA 2
Midwest Commercial Small Fruit and Grape Spray Guide PM 1375
Extension Distribution Center
119 Printing and Publications Building
Iowa State University
Ames, IA 50011
This article originally appeared in the 7/25/2003 issue.