Every year I get calls into the Extension office from grape growers wondering what happened to their promising grape crop. The conversation generally goes something like: “I had a great looking grape crop and then some of the grapes started to develop a little brown spot that spread over the entire grape. I now have bunches of shriveled up grapes hanging from my grape vines. What happened?”
This is a classic description of grape black rot symptoms. Black rot is a fungus disease that will develop and thrive most years here in Ohio. Unfortunately, by the time you see the disease symptoms, it is often too late to do anything about it. This is a disease that starts in the early spring after bud break as shoots and canes begin to grow. The black rot fungus overwinters in those shriveled up mummy berries and spores are released during spring rains to infect new tender tissues. Spores need wet plant tissue and some minimum temperature to germinate and penetrate into the tissue. When temperatures are in the 60 to 70 degree range it requires 9 hours or less of wet plant tissue to get infection.
The key to control involves a multi-pronged approach consisting of sanitation and timely fungicide application. Sanitation involves cleaning up the shriveled “mummy” berries and destroying them. It is very important that not to leave any mummy berries attached to the vine because they can serve as an infecting source of spores throughout the growing season and make control with fungicides more difficult and less effective. Cleaning up as many as possible of the mummy berries on the ground is also important to reduce the amount of spores released in the early spring period.
If the grapes are not located in an open, sunny area, this will make black rot disease control more difficult. Anything that can be done to promote more sunlight penetration and better air movement will aid in disease control. The final prong of black rot control is the regular and timely application of an effective fungicide.
To control black rot, fungicide application should begin when new canes are 3 to 5inches long and should be repeated on a 7 to 10 day interval through 4 to 5 weeks after bloom. For most growers, commercial and backyard alike, Mancozeb should be the backbone of this fungicide application program.
Mike Ellis, professor emeritus, and longtime OSU Extension Fruit Disease Specialist, has put together some very helpful documents to help grape growers control diseases in grapes including black rot. I am also going to include a good simplified spray guide from the University of Kentucky Extension for home grape growers. Click on the links below for more information about controlling grape black rot including specific fungicide recommendations: