Break out the baking soda and spray away your powdery mildew woes. Cornell University discovered baking soda was an eco-friendly effective way to control powdery mildew. Mix 1 tablespoon bakingsoda and one teaspoon insecticidal soap or light weight horticulture oil (these act as spreader-stickers) into a gallon of water. Monitor mildew susceptible plants throughout the season. Start applying this mixture when the disease first appears. Repeat weekly to keep this fungal disease in check.