Mix up a spray with 1 tablespoon organic washing up fluid, and 2 teaspoons of baking soda, in a gallon of water. Spray thoroughly and frequently. The washing up fluid kills blackflies on contact, but has no residual action, so any you miss will repopulate the plant in less than a week. The combination kills the spores of the mildew--the white powder will remain, but the new growth should be free of it. Regular feeding with a high potash food will also help the plants to resist infection and infestation.