The Q&A Archives: Fruit problems

Question: Whats the best way to stop black rot on grapes?

Answer: Black rot indeed can be frustrating to deal with, but home gardeners can keep the disease at bay by using a spraying regime. Black rot is a fungus disease that overwinters primarily in mummified fruit. You can reduce the spread by being scrupulous about removing all mummified fruit and infected leaves and stems at the first sign of disease. At the end of the season, rake up all fallen leaves and fruit and dispose of them.

Infections begin to appear on the fruit when the grapes are about one-half grown. At first a small white spot forms on the surface of a grape berry. This spot enlarges rapidly until the entire grape is rotten. Affected grapes soon turn black, shrivel, and dry up. Within a few days most of the grapes within a cluster are infected and soon rot.

How do you handle black rot? Make sure you're spraying vines with a recommended fungicide every ten to fourteen days. Fungicides which do a good job of controlling black rot include captan, Benlate, and Bordeaux mixture. Regardless of the fungicide selected, it's important to apply using the recommended rate and at the timing interval recommended on the fungicide label.

Another factor which influences control of black rot deals with fungicide coverage. Obtaining coverage of the foliage and grape clusters with spray material is the most important part of the entire spray program. The large leather-like grape leaves are difficult to move around with spray material. Consequently, many bunches of grapes are not adequately covered. To control black rot, every grape cluster must be completely covered with fungicide each time the vines are sprayed.

If you continually have problems, you may want to switch to one of the disease-tolerant varieties, such as Catawba, Concord, Chancellor or Riesling.

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