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Plant Pathology Infobytes

May 24, 1996

Time To Handle Black Rot Of Grapes

Tired of having high expectations for your grape crop only to lose the entire harvest to blight and rot? If you've had grape disease problems in past seasons, chances are the culprit was black rot. Black rot is the number one disease which affects bunch grapes in home vineyards and frequently destroys an entire fruit crop within a few days.

Now's the time to really concentrate on a control program to take care of black rot. There's another plus for keeping this disease in check, since other diseases which affect bunch grapes will be less troublesome when recommended practices for black rot control are carried out.

Black rot is caused by a fungus which invades all parts of the grape vine, causing symptoms on stems, tendrils, leaves and fruit. Leaf infections appear as tiny, reddish-brown spots on the upper surface in early June. These increase in size and eventually blight the entire leaf.

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.

Check with us at your county Extension office for further informa-tion and copies of free publications on disease control.

Infobytes newsletter was written by the late Dr. Frank Killebrew, Extension Specialist.