To treat a plant for powdery mildew, mix 1 tablespoon of baking soda and 2 1/2 tablespoons of horticultural oil in a gallon of water, and apply the mixture to the plant. Commercial preparations of baking soda, sulfur, beneficial bacteria or neem oil are also available. Use the product according to the instructions on the package.
Alternately, mix 1 teaspoon of baking soda in a quart of warm water, and apply the mixture to the plant every seven to 10 days. Another option is to dilute 3 tablespoons of apple cider vinegar in a gallon of water, and apply the mixture to the plant weekly. If gray mold is present, remove the diseased foliage and spray the plant with a beneficial bacteria.
To help eliminate and prevent powdery mildew and gray mold, prune the plant to promote adequate air flow, and remove nearby weeds if the plant is growing outdoors. Water the soil, rather than the plant's leaves, and avoid using overhead watering systems.
Water the plant early in the morning to ensure the leaves dry quickly during the day. Do not use fertilizers that are high in nitrogen if powdery mildew is present. Instead, use a balanced fertilizer that is rich in potassium and phosphorous.