Powdery mildew gets its name from the white, powdery spots which appear on both the top and the bottom of leaves. It can also be found on the older vines. Downy mildew can be recognized as small, irregular yellow spots on leaves, which grow and turn brown. Underneath the leaves, a fine purplish mold can be found early in the morning. Which disease is more of a problem in a given year depends on the weather.
Downy mildew is more common in wet years.
Powdery mildew will occur during warm and humid or dry periods, but not when the weather is wet. Early in the season, a mixture of paraffinic oil and baking soda can be used to control powdery mildew at the rate of three tablespoons each per gallon of water. Spray the mixture twice a week on the top and bottom of leaves. SunSpray is one manufacturer of paraffinic oil. Chlorothalonil is only a protectant, so it must be applied before the fungus infects the leaves or fruit.