Got grapes? If so, you may be well aware that black rot, caused by the fungus Guignardia bidwellii, is the most serious disease of cultivated and wild grapes. If you are not familiar with grapes, you may want to brush up on this important disease. Although we have not seen black rot at the Plant Clinic this season, it has been reported in neighboring states.
The disease is most destructive during warm, wet seasons. The fungus can infect all parts of the vine—the shoots, leaf and fruit stems, tendrils, leaves, and fruit. However, the most damaging effect is on the fruit. Infections early in the growing season destroy the blossom clusters, while later infections can rapidly destroy many of the berries.
Starting in late spring, you may notice reddish brown and circular to angular lesions on the upper surface of the leaves. The center of the lesion turns tan to brown and has a black border. Lesion size is variable, and lesions will merge to form large blotches. Only young, rapidly growing leaves are susceptible to infection.
Although fruit can be infected shortly after petal fall, most infections occur when the green fruit is half to almost full size. A small spot appears that is circular and whitish tan, often surrounded by a brown ring. The spots rapidly enlarge, darken, and then become sunken and wrinkled. Eventually, the berry becomes coal black, hardens, and drops early. The surface of the withered fruit is covered with minute, black, pimplelike fruiting bodies that are often arranged in circular zones. This entire process takes only a few days.
Lesions on other parts of the vine are sunken, dark purple to black, and are oval shaped or elongated. Fruiting bodies are scattered throughout the lesion. These lesions can cause splits in the cane bark. Early berry stem infections can cause berries to shrivel and fail to develop.
Black rot is not difficult to control if the following management practices are used.
• Choose grape cultivars that show resistance to black rot and other important grape diseases. A list of cultivars is presented in Report on Plant Disease No. 703, “Black Rot of Grape,” available through your local U of I Extension office or on the Web at http://www.ag.uiuc.edu/~vista/horticul.htm.
• Promote rapid drying by choosing a sunny planting site, properly spacing and tying the vines, and removing weeds and tall grass.
• Prune properly during the dormant season, and destroy diseased tissues to reduce inoculum of the fungus, thus limiting disease.
Use fungicide sprays during wet seasons to protect the developing growth. Follow the grape spray schedule outlined in either Circular MD-1, Illinois Commercial Small Fruit and Grape Spray Guide or, for the homeowner, the Illinois Homeowners’ Guide to Pest Management. These publications are also available through your local U of I Extension office.