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Sandy's Garden

Grapevine Black Rot

When grapes don't ripen properly it could be black rot disease

By: Sandy Feather ©2009
Penn State Extension


Q. I believe my grapevines have black rot.  The concord grapes develop, but before they completely mature, they shrivel up and fall off.  I have been treating them with Captan.  I wanted to know if this is the proper treatment and if there is a cure for this disease. This has been going on for about four years. 
  
  A. Your description of the grapes shriveling up before they are mature enough to harvest is classic for black rot. Black rot is perhaps the most common and serious fungal disease of grapes in the eastern United States.

It is caused by the fungus Guignardia bidwelli, and infects leaves, green stems and developing fruit. The entire crop may be destroyed. It is most severe during wet weather.

Mixed fruit with grapes

Symptoms

Symptoms of black rot include reddish-brown circular spots on the upper surface of the leaves. The center of the spot turns light brown, and small black fruiting bodies (the size of a pinpoint) appear just inside the darker border. These fruiting bodies produce spores that continue to infect other portions of the plant, including the maturing fruit. Infected grapes may become dark and shatter, leaving only the stem. Or they may shrivel and remain attached to the stem. These are referred to as “mummies.”

Life Cycle

Black rot fungus overwinters on mummified grapes on the plants or on the ground near the vines. Fungal spores are produced in the mummified fruit during spring rains. These spores infect leaves, blossoms and young fruit. The fruit may be infected from blossom time until they begin to color. The severity of the infection depends on temperature and the length of time the leaves are wet. Infection is most severe when we have wet, mild (70 - 80°F) weather. Mature leaves and fruit are not susceptible to black rot. Usually, little infection occurs after late July, and none after late August.

Although it is too late to save this year’s crop, there are steps you can take to improve next year’s harvest. Good garden sanitation is one important way to minimize black rot infections on next year’s crop. Remove all mummified grapes and rake up fallen leaves in autumn. Dispose of infected plant material by burning it or sending it out with the trash. Avoid composting infected plant material. It is also important for you to prune your grapes next spring. Grapes should be pruned every year to clean out dead wood, which will remove fungal spores overwintering on the vines. Proper pruning also permits good air circulation through the vines and makes it easier to get thorough coverage when you apply fungicides.

Fertilization

Fertilizer applications should be based on soil test results, rather than guessing what nutrients the grapes need for maximum production. Too much nitrogen can force succulent growth that will be more susceptible to infection.

Grapes

Fungicides

Fungicide applications should be made on a strict schedule next spring as soon as new growth begins. Applications to control black rot should be made every 14 days, beginning at bud swell. That interval should be shortened to every 7 - 10 days during long rainy periods. Spray in the rain if necessary to stay on schedule. Applications should continue until 10 - 14 days before harvest. You may use a home orchard spray that contains both an insecticide and a fungicide. However, you cannot spray the combination product during bloom or you will kill pollinators such as bees. You may also choose to use a fungicide alone to control this disease so that you do not have to skip any sprays to protect pollinators. A mix of Captan plus Benomyl is labeled to control black rot on grapes.  Organic gardeners may use a sulfur-based fungicide, but it is not as effective against black rot as the combination of Benomyl and Captan. Also, sulfur applications may injure plants if temperatures exceed 85°F. Always read and follow label instructions as to application rate and timing of sprays.

Resistant Varieties

When available, growing resistant varieties of problem-plagued plants is always helpful. Unfortunately, very few grape varieties that are hardy in our climate exhibit resistance to black rot. Research from the University of Kentucky found two varieties of table grapes reported to be resistant. Mars, a blue seedless variety, was rated resistant. Remally, a white seedless variety was rated moderately resistant. Both varieties are recommended for planting in Pennsylvania, according to Penn State's Small Scale Fruit Production Guide.  The guide also contains information on proper pruning and other disease and insect problems grape growers may encounter. You can find Penn State's Small Scale Fruit Production Guide on the Internet at:  http://ssfruit.cas.psu.edu/ 


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