Powdery mildew is one of the most commonly occurring plant problems. It is a fungal disease that affects plant leaves and stems, coating them in what looks like a white or gray powder-like substance.
Combine one tablespoon baking soda and one-half teaspoon of liquid, non-detergent soap with one gallon of water, and spray the mixture liberally on the plants. Since Powdery mildew stays in the soil via spores, you may want to change your garden location to a different area next year.