In Oklahoma, black rot, caused by the fungus Guignardia bidwellii, is the most important foliar disease of grapes.


In Oklahoma, black rot, caused by the fungus Guignardia bidwellii, is the most important foliar disease of grapes.
Black rot can be managed using cultural practices and chemical control. However, last season’s abnormally large amount of rainfall and mild temperatures made for a challenging season for managing pathogens in the vineyard. This included black rot.
Many growers found it difficult to get into the field to make fungicide applications, or they found that their “resistant” variety wasn’t so resistant when the weather was continuously favorable for disease development. As a result, a significant level of disease was present in many vineyards.
This has implications for this season, too. All of last year’s disease has resulted in the production of large quantities of primary inoculum available for production of new infections this season.
As many varieties are beginning to approach cap fall, and flowering this season, it is important that growers and home gardeners continue to focus their attention on protecting their crop from infection by the black rot fungus.
Black rot will manifest on leaves, petioles and canes initially, followed by secondary infections of fruit. Most loss is a result of direct destruction of the fruit by the fungus. Genetic resistance is the easiest and most economical method of management for this disease. Most Vitis vinifera grapes are highly susceptible to G. bidwellii, while many French-American hybrids and American varieties have better resistance to the fungus that causes black rot.
Fungicides are available for managing the disease. Fungicides should be used preventatively when weather is predicted to be favorable for infection and disease development.
Infections by the fungus are driven by a combination of continuous leaf wetness and specific temperatures. Temperatures of 70 degrees Fahrenheit to 80 degrees Fahrenheit require the smallest duration of leaf wetness and are the most favorable temperatures for fungal infection.
In Oklahoma, preventative applications of fungicides should begin when shoots are  three to 10 inches in length and should continue at regular intervals.
The most critical time for application of fungicides is just prior to bloom until at least four to six weeks post-bloom.
Once veraison is initiated, natural resistance in the fruit exists and fungicides are not required.
Growers should not forget about the presence of other pathogens in their vineyard however. Chemical management of powdery mildew must continue until harvest and may even require an application of fungicide postharvest.
Spray intervals should also be shortened if extended periods of wet weather are forecasted. Fungal resistance to fungicides should also be considered. Fungicides should be rotated frequently to minimize the occurrence of fungicide resistant populations of pathogens in the vineyard.
In addition to chemical management, growers should use good cultural practices in their vineyards to prevent black rot. Proper canopy management and sanitation, including removal of mummies from the canopy and cultivation or burial of debris can help limit damage caused by the black rot fungus.
If you have not been spraying, getting on a spray schedule for grapes in Oklahoma is a must do. Spray schedules for grapes may be obtained at the Extension Office. Using a spray schedule this season will be helpful in control of disease next season.
If you have questions concerning this topic or related topics, call the OSU Extension Center at 273-7683, stop by the office at 14001 Acme Road in Shawnee or visit our Web site at http://countyext.okstate.edu/pottawatomie/.