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Save grapes from fungal disease with thorough cleanup

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Jessica Walliser
Healthy grapes on a grape vine.

Question: Each year, I struggle with black rot on my grapes. I have used Serenade but without much success. Can you suggest any methods to eliminate the problem? Perhaps a different fungicide, a different way to use Serenade, or maybe something I can do to the soil beneath the grape vines?

Answer: Black rot is a fungal disease, caused by the fungus Guignardia bidwellii, which strikes grapes with great frequency, especially during wet conditions. If not controlled, many half-grown grapes can rot and fall off the vine.

Upon infection, the initial symptoms of black rot are yellow spots on the leaves. Eventually, these spots turn dark around the margin, and two weeks later, they disperse thousands of spores. Dark lesions can occur on stem tips and tendrils as well. These spores quickly spread onto developing fruits and begin to infect them.

Infected fruits exhibit small, round, brown spots that quickly expand, eventually causing the entire berry to rot within a few days. When it is rotted, the diseased fruit shrivels and turns hard and black. These grape “mummies” contain thousands of spores that can go on to infect more fruit clusters and plants.

Black spot easily overwinters on these fruit mummies and on fallen grape leaves and other debris that may contain spores. When spring arrives again, the fungus becomes active, and a new cycle begins.

Black rot can be difficult to control. No fungicide will provide 100 percent control, unless you also follow a few cultural practices to help control this disease.

Of critical importance: Every fall, clean up the vines. Destroy all grape mummies by burning them or tossing them into the garbage. Mummies that fall to the ground or stay attached to the vine are huge vectors for reintroduction of spores the following season. They must be removed. Remove diseased leaves by raking them away from the plants and throwing them away.

A dormant spray of lime-sulfur during late winter also will help control overwintering spores, but it’s not a substitute for cleaning up the orchard and disposing of all mummies.

Grapes should only be grown where air circulation is good. To improve air circulation and help the developing fruits dry off faster after rains, summer pruning is essential. In late June or early July, head out to the orchard with a clean, sharp pair of pruners and a cup of 10-percent bleach solution. Cut off all grape leaves, shoots and tendrils that hang over any clusters of developing grapes. The fruits should be exposed to sunlight and air as it grows. Dip the blades of your clippers into the bleach solution between every cut to ensure you aren’t spreading any pathogens from one plant to another.

Biofungicides, such as Serenade, are effective against black rot when used appropriately and in conjunction with the cultural practices I outlined above. Vines should be sprayed well before any symptoms occur, especially if you’ve dealt with this fungus before. You need to start spraying as soon as the initial vine growth reaches about 2 to 3 inches long because it’s important to control the pathogen before it becomes established on the leaves and spreads to the fruit. Apply to both upper and lower leaf surfaces as well as to the vines themselves.

Continue spraying at 10-day intervals until four weeks after the plants finish blooming. This is the most critical time for control. Spraying early, before the plants come into flower, is very important for controlling this disease.

After the grapes form and they reach four to five weeks old, they’re naturally resistant to the fungus that causes black rot, and sprays are no longer needed.

Horticulturist Jessica Walliser co-hosts “The Organic Gardeners” at 7 a.m. Sundays on KDKA Radio with Doug Oster. She is the author of several gardening books, including “Attracting Beneficial Bugs to Your Garden: A Natural Approach to Pest Control” and “Good Bug, Bad Bug.” Her website is jessicawalliser.com.

Send your gardening or landscaping questions to [email protected] or The Good Earth, 503 Martindale St., Third Floor, D.L. Clark Building, Pittsburgh, PA 15212.

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