This is straight from the bible.
-Powdery mildew-
Identify: First indication of infection is small spots on the tops of the leaves. At this point the disease has been inside the plant a week or more. Spots progress to a fine, pale, gray-white powdery coating on growing shoots, leaves and stems. Powdery mildew is not always limited to the upper surface of foliage. Growth slows, leaves yellow and plants die as the disease advances. Occasionally fatal indoors, this disease is at its worst when roots dry out and foliage is moist. Plants are infected for weeks before they show the first symptoms.
Control: Cleanliness!! Prevent this mildew by avoiding cool, damp, numid, dim grow room conditions, as well as fluctuating temperatures and humidity. Low light levels and stale aire affect this disease. Increase air circulation and ventilation, and make sure light intensity is high. Space containers far enough apart so air freely flows between plants. Allow foliage to dry before turning off lights. Remove and destroy foliage more than 50% infected. Avoid excess nitrogen. Copper and sulfur-lime sprays are a good prophylactic.
Biological Control: Apply serenade (bacillus subtilis) or spray with a saturation of baking soda and water.
Sprays: Bordeaux mixture may keep this mold in check. A saturation of baking soda spray dries to a fine powder on the leaf; the baking soda changes the surface ph of the leaf to 7 and the powdery mildew cannot grow.
Other than that sorry I can not give my own advice, I am only 2 months into my first garden, im still a noob!