As we begin to think about planning for the growing season, one of the questions that seems to come up quite a bit is how to get rid of black rot on grape vines. Unfortunately, like many gardening headaches, the answer is not simple.
Black rot is caused by a fungal pathogen, Guignardia bidwellii. The fungus thrives under warm, humid conditions that are typical of an Ohio spring and summer, which means black rot will be a problem most years.
So how do you know you have black rot on your grape vines? The key diagnostic feature of black rot is a small, brown lesion that begins on the fruit and grows over the season until the grape shrivels into what is called a “mummy”. These mummies may fall off of the vine or remain attached to the plant — either way, these mummies provide inoculum for the next season. Leaves can also develop lesions as well — small, black dots (called pycnidia) may appear on lesions that contain the summer spores. The fungus overwinters in lesions and mummies.
One of the best ways to control black rot is to look at site selection. Spores of the fungus are typically spread during periods of rainfall that keep leaf surfaces wet, but some spores are wind dispersed and can travel long distances to new host plants. Make sure vines have adequate access to sunlight and good air circulation to reduce risk of initial infection. Black rot spreads rapidly when excess vegetation (leaves) are present. Pruning is a good way to increase sunlight and circulation among leaves. Without the right conditions for disease, black rot should not become a problem.
If you already have black rot in your grapes, remove mummies and prune infected leaves. You can apply fungicides for control — always follow the label. These applications should be made when canes are 3-5 inches long and repeat every 7 to 10 days until about a month after bloom. Mancozeb and Ziram are both effective against black rot but are protectants (meaning they should be applied before infection occurs.) If you have lesion development, it is too late to get adequate control from these fungicides.
Several Sterol Inhibiting (SI) fungicides provide good control of black rot if applied according to the label within 3-4 days after infection (before symptom appearance). Nova (now Rally) or Immunox, Elite, and Procure are SI fungicides used for black rot.
Unfortunately, as with many pathogens, when you begin to see symptoms, it’s often too late to get adequate control. Prevention is always the best management practice.
Above: Lesions produced by G. bidwellii on leaves (left) and black rot mummies on a grape vine (right). Photo credit: University of Georgia Plant Pathology , University of Georgia, Bugwood.org.
To learn more about management of black rot, check out the following resources:
Controlling Grape Black Rot in Home Fruit Plantings
Critical Periods for Fungicide Application
Developing an Effective Spray Program for Commercial Wine Grapes
Grape Black Rot Fact Sheet
Homeowner Control of Grape Black Rot
Midwest Fruit Pest Management Guide
Simplified Backyard Grape Spray Schedule