Here’s a tip you might find interesting. Researchers in Canada have discovered that household Palmolive detergent both prevents and fights powdery mildew symptoms on fruit.
It was as good as the registered chemical product for commercial control with one important exception. It caused fruit damage at the rates tested (russeting on apples and grapes, premature dropping of cherries).
NOTE: This research was done on fruit trees and these have a different (tougher) structure than many perennials or annual plants. Before spraying anything but fruit trees – do your testing! Or you’ll burn the devil out of your plants. And test only one or two leaves – not the entire plant.
If you want to test this – start with .05% of detergent in water and increase the concentration until you see control or damage. There are simply too many variables here to judge what’s going to work and what’s going to damage plants. (That’s 5 ml per litre of water – not much at all)
There are unconfirmed reports that some gardeners have combined sodium bicarbonate (baking soda which is often recommended for pm control) with the detergent to give a broader spectrum of control at lower doses. Again, this is a test and trial kind of thing.
What is clear is that if you go and spray at heavier concentrations, you run the risk of burning or damaging plants.
There is room here for experimentation but like all things, do take precautions to avoid breathing the small droplets of spray (wear appropriate masks)
Reference: Peter L. Sholberg (Pacific Agri-Food Research Centre, Agriculture and Agri-Food Canada, Summerland, British Columbia, CANADA V0H 1Z0), Palmolive Detergent Controls Apple, Cherry, and Grape Powdery Milidew,Canadian Journal of Plant Science 89(6), November 2009, 1139-1147.