I've had excellent results with Mildew Cure. It pretty much eradicates powdery mildew on all my cucurbits. It's OMRI listed, but the downside is that it's based on cottonseed oil (30%) which is not organically grown, therefore grown with extremely toxic chemicals; it also contains garlic oil (23%) and clove oil (30%) for which the oil base isn't listed on the label, but Johnny's catalog suggests it's corn oil, which again, is not organic and therefore extremely likely to be GMO.

The Mildew Cure label lists sodium bicarbonate as an inert ingredient; but after I bought it three years ago (it works really well and goes a long way!) I've seen lots of recipes for mildew control using baking soda and hort oil only, and studies confirming that it's effective. Typical recipes call for 3-4 tsp of baking soda and 2-2.5 tbsp of horticultural oil (any lightweight vegetable oil - corn or safflower e.g.) in a gallon of water. Sometimes they'll add 1/2 tsp of dish soap as a surfactant as well.