Plant Pathology Infobytes

May 27, 1998
Don't Lose Your Grape Crop to Black Rot

Tired of having "grape" expectations only to lose your crop to fruit blights and rot? Now's the time to take action against black rot and the other diseases which reduce grape fruit yield and quality.

If you are an orchardist who has experienced disease in your grape crop, chances are the culprit was black rot. Black rot is the number one disease which affects bunch grapes in home vineyards and this fungal problem frequently destroys an entire fruit crop within a few days.

Now's the time to concentrate on a control program to take care of this disease. There's another plus for keeping black rot under control, because other diseases which affect bunch grapes should be less of a problem when recommended practices for black rot control are followed.

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 the County Extension Officecounty Extension office for further information and copies of free publications on disease control.


Written by the late Dr. Frank Killebrew

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