Leave and Foilage Problems
Tan leaf spots with a dark border
Cause
Black Rot (Guignardia bidwellii), a fungal disease which overwinters in crop debris and is spread by wind and rain.
Treatment
Plant disease resistant varieties whenever possible. Clean up crop debris regularly, including fallen berries. Bordeaux mixture may be used for black rot.
Yellow and orange leaf blotches with gray mold
Cause
Downy mildew (Plasmopara viticola), a fungal disease which overwinters in crop debris and is worst during hot humid weather (June-August).
Treatment
Plant disease resistant varieties whenever possible. Clean up crop debris regularly, including fallen berries. Bordeaux mixture is approved for black rot.
Powdery white mold
Cause
Powdery mildew (Uncinula necator), a fungal disease which is most common during cool, humid, and windy conditions.
Treatment
Apply copper or sulfur-based fungicides at the first sign of powdery mildew.
Fruit Problems
Shriveled, black berries with brown spots
Cause
Black Rot (Guignardia bidwellii), a fungal disease which overwinters in crop debris and is spread by wind and rain.
Treatment
Plant disease resistant varieties whenever possible. Clean up crop debris regularly, including fallen berries.
Wrinkled, brown berries with white mold
Cause
Downy mildew (Plasmopara viticola), a fungal disease which overwinters in crop debris and is worst during hot humid weather (June-August).
Treatment
Plant disease resistant varieties whenever possible. Clean up crop debris regularly, including fallen berries.
Gray fuzzy mold
Cause
Gray mold (Botrytis cinera), a fungal disease.
Treatment
Disease symptoms are often worse in cool, wet conditions. Avoid excessive irrigation, clean up and remove crop debris. Resistant varieties, and approved fungicides are available.
Shriveled berries covered with insect webbing
Cause
Grape berry moth (Endopiza viteana), the small caterpillar of a moth which spins webbing on grape flowers and fruit as it feeds.
Treatment
Inspect flower clusters several days before bloom, re-inspect the fruit clusters immediately after the bloom, and again in early August when a second generation of the moth sometimes occurs.