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Dealing With Grape Diseases

Sunday, February 25, 2024

In my last column, I talked about the process of growing grapes. For those who already have grape vines established, grape diseases can cause serious problems.


One such disease gardeners need to be aware of is black rot, which is a foliar and fruit disease caused by the fungus Guignardia bedwellii. It can result in the loss of the fruit if gardeners don’t get it under control.


Black rot usually presents as small, whitish areas on the grapes themselves before evolving into a widening brown ring. The rotted fruits shrivel and turn black. These rotted fruits, known as mummies, contain spores of the disease that are released the following spring as the temperature begins to rise and the fruit is wet.


One thing that will help control the disease is pruning the vines of the infected tissue. The prunings should be removed from the site and burned. Don’t compost them. Another approach is shallow cultivation of the soil beneath the vine. This helps cover the mummies and prevents the release of the spores. Keeping the foliage dry also aids in the prevention of the disease. Leaf wetness is a key element in the spread of black rot.


Even with using cultural controls, black rot can be difficult to control without a fungicidal spray program. Isolated vines in a home garden may not have problems with black rot, so a spray program may not be needed; however, if vines have had a problem with black rot in the past, a spray program may be warranted to get the disease under control.


Starting early in the season is key to gaining control over the disease and may make later sprays unnecessary. The first spray should be made in the dormant season, which is now through mid-March. Apply a second spray when the new shoots have two leaves. A third spray can be applied when the new shoots are 12 to 18 inches long, and a fourth spray should be applied after petal fall.


Don’t spray any pesticides during bloom as this can be harmful to bees and other pollinators.


More information about black rot can be obtained from the local Oklahoma State University Extension county office. Also, check out OSU Extension’s Black Rot of Grapes fact sheet.

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