Plant Disease Clinic -- Highlights

News Article

Black Rot of Grape

Black rot of grape is caused by the fungus Guignardiabidwellii. On leaves, the first symptoms are small, yellowlesions. These spots develop into brown lesions that aresurrounded by dark irregular margins. Fungal fruiting structures(pycnidia) form within these spots and appear as small, black dots.

Purple to black sunken lesions may also develop on youngshoots and stems. Fruiting structures are also evident in theselesions. Fruit are susceptible to infection. First, smallbrownish spots appear on fruit. These spots enlarge very quickly,often within a few days. The infected fruit then shrivel andbecome hard and black. These diseased fruit are referred to asmummies.

The black rot fungus can survive the winter in mummifiedberries and also in stem lesions. During wet spring weather thefungus is spread to new tissue. Sanitation measures are thereforevery important in controlling this disease. All mummies anddiseased stem tissue should be removed. Because the disease isfavored by wet conditions, good air movement among plants ishelpful in preventing the disease. Chemical control is based onthe use of protective fungicides. Home orchard sprays areavailable to homeowners. Make sure the product is labeled forcontrol of black rot of grape. The product label will giveinstructions for proper timing of applications, rates, and safetyprecautions. Commercial growers can refer to Pm-1375 "IowaCommercial Small Fruit Spray Guide".

This article originally appeared in the August 11, 1993 issue, p. 136.