This could be one of those rare years where we see downey mildew in grapes. DM is rare in the SJV because of our normally low humidity. DM is first observed as small, pale yellow to reddish spots with indefinite borders on the upper leaf surface with white downy growth on the lower surface. Infected leaves are killed and turn brown and may defoliate. Young infected shoots and cluster stems may curl, distort, show thickening of the infected tissue and may be covered with a whitish fungal mass.
Infected berries turn brown and eventually shrivel.
Infective spores are produced during rainy periods if temperatures are above 50F and infections are triggered by warm, humid nights followed by rain. This disease makes rusty spots which look very much like iron rust on the under side of leaves, and the tops of the leaves show yellow color to match the rust spot below. Rust can cause severe defoliation. Sulfur is a very effective control if applied before, or at the first sign of rust. Almond scab , caused by another fungus, Fusicladium carpophilum , makes greasy gray spots on the leaves and on the tops of the nuts.