Q. This season a lot of my grapes turned black and dried up. Please advise.
It sounds like your grapes have black rot, a fungal disease caused by the pathogen Guignardia bidwellii. This fungus attacks all green parts of the plant – leaves, shoots, leaf and fruit stems, and tendrils – but is most damaging to the fruit. The fungus is most active and destructive when spring and summer weather is warm and wet.
Early signs of the disease are reddish brown and circular to angular spots on the upper surface of young, rapidly growing leaves starting in late spring. Shortly after the flower petals fall, the fruits can become infected. Small circular, whitish tan spots appear, often surrounded by a brown ring spot, when fruits are half to almost full size and still green. These spots grow rapidly and may cover half of the berry within 48 hours. Within a few days the entire berry becomes coal black, hard, and mummified. The surface of the withered fruit is soon covered with black, pimple-like, spore-producing organs.
These mummified berries (on the ground or still clinging to the vine) are the major source of infection the following spring. The microscopic spores are propelled by rainfall into air currents, which carry them to the young, expanding leaves. In the presence of moisture, these spores germinate in 36 to 48 hours and eventually penetrate the leaves and fruit stems. When the weather is wet, spores can be released the entire spring and summer providing continuous infection. Cool weather slows growth of the fungus.
Black rot cannot be cured once the grapes are infected, but there are a number of integrated pest management strategies for this disease.
- Space vines properly; plant where they will be exposed to full sun and good air circulation. Keep vines off the ground and insure they are properly tied.
- Keep the area free of weeds and tall grass to reduce humidity.
- Prune the vines in early winter during dormancy. Select only a few strong, healthy canes from the previous year’s growth to produce the following season’s crop. Remove the pruned parts and burn or destroy them.
- Bury the mummified berries before budbreak, using 2 – 3 inches of leaf mulch or fine bark to cover infected debris.
- Use protective fungicide sprays. Pesticides registered to protect the developing new growth include copper, captan, ferbam, mancozeb, maneb, triadimefon, and ziram. Important spraying times are when new shoots are 2 to 4 inches long, and again when they are 10 to 15 inches long; just before bloom; just after bloom; and when the fruit has set.
- Cultivars with large, juicy berries are the most susceptible. In general, grapes that ripen late in the season are affected the least. ‘Baco #1’, ‘Cascade’, ‘Cayuga White’, ‘Chancellor’, ‘Chelois’, ‘DeChaunac’, ‘Elvira’, ‘Missouri Riesling’, ‘Norton’, ‘Vidal 256’, and ‘Verdelet’ are moderately resistant varieties.
Grapes are susceptible to a number of other diseases. The page linked to below describes their symptoms and management strategies, in the event that you look into black spot and determine that it is not what's affecting your plants.
Courtesy of NYBG Plant Information Service
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