Hey there, here is my advice to prevent and to kill powdery mildew. I spray my plants plants once a week with milk to prevent the fungus from growing on my leaves.The milk contains lacto bacillus and other bacillus strains the compete with the fungus, I apply at a rate of 1.5 cups milk to 1 gallon of water. If I do get an infestation I apply 1 tablespoon per gallon of water and apply in the morning. Hope this helps