I sprayed my plants with a pump sprayer today with 1 gallon of well water with a tablespoon of potassium bicarbonate.
Mix 4 teaspoons (about 1 rounded tablespoon) of potassium bicarbonate into one gallon of water. Spray lightly on foliage of plants afflicted with black spot, powdery mildew, brown patch and other fungal diseases.
Potassium bicarbonate– Similar to baking soda, this has the unique advantage of actually eliminating powdery mildew once it's there. Potassium bicarbonate is a contact fungicide which kills the powdery mildew spores quickly. In addition, it's approved for use in organic growing.
1 pound for $8.99 at Amazon.
You can try H2O2 mixed with water in a pump sprayer but I would go with potassium bicarbonate. Growtogrow mentions it in his outdoor grow thread.