I beg to wade in on this debate to respectfully submit the following: "Safer Insecticidal Soap" is an insecticide, excellent at killing aphids in particular, etc. It is made from potassium salts of fatty acids, it is useless against powdery mildew, nor does the manufacturer make that claim. Safer does offer a product called "Defender" which contains sulphur, the best and safest fungicide available. There are tons of other providers with similar "sulphur" based spray products. And while on the subject of "powdery mildew", with all due respect to previous posters, Neem Oil is not intended to prevent "powdery mildew" in spite of what some manufacturers may claim. It's main claim to fame, is in clogging the lungs of some insects which attack fruit trees, and roses, applied before bud out, but nothing to do with "powdery mildew". I have used all these products mentioned above for years, except that I buy powdered Sulphur, and mix it in with my spray container - note, talking a lot of roses and fruit trees. Any of the 1L hand bottle sprays containing Sulpher, work well. I have used sodium bicarbonate, but it is decidedly inferior to sulphur. Cheers!