This plant disease is caused by a fungus (Guignardia bidwelli). It is the most destructive disease that attacks grapes, often destroying all the fruit. The fungus spends the winter in infected dormant canes, tendrils on the support wires, and mummified fruit. In warm, moist spring weather, spores infect new shoots, leaves, tendrils, and eventually the developing fruit. The fruit is affected in all stages of development, but most severely when it is one-half to two-thirds grown. Spores for future infections are produced on infected leaves, canes, and fruit. The severity of the disease depends on the amount of diseased material that survives the winter, and on the spring and early summer weather.
Destroy all infected fruit and prune out infected canes and tendrils. Once the fruit has begun to shrivel, fungicide sprays are ineffective. Next year, spray the plants with a fungicide containing captan. Spray early in the growing season to keep the spread of the disease to a minimum. Treat when new shoots are 6 to 10 inches long; just before and immediately after bloom; and continue at intervals of 10 to 14 days until the grapes are full size. Treat more frequently if leaf symptoms develop, or if the weather is wet. For grape varieties resistant to black rot, see Disease Resistance of Grapes.
Plant Care for Grapes