Mix 4 tsp. of baking soda into a gallon container that is filled with water. Add up to 2 1/2 tbsp. of a horticultural oil if desired, to prevent and treat other pests or diseases that may be affecting your plant. Horticultural oil can help control moderate and more severe infections of powdery mildew, according to Mary Louise Flint, director of the University of California Integrated Pest Management Education and Publications.
Pour some of the baking soda and water mixture into a spray bottle.
Spritz the leaves that are affected with powdery mildew with the baking soda spray. Take care not to drench the plants; the Royal New Zealand Institute of Horticulture explains that too much of the liquid can cause damage to the leaves and to the surrounding soil.
Treat the plants again with the baking soda deterrent as needed when mildew forms on the plant again. The University of Wisconsin suggests controlling powdery mildew with baking soda on a weekly basis.