How can I treat powdery mildew which is rampant in my garden this year?
Multnomah County Oregon powdery mildew
Thanks for your question about powdery mildew control. You don't say what plants you're finding it on (ornamental or vegetable). So I'm going to assume it's both types of plants, and that you'd like information about both chemicals (that often kill beneficial insects, too) and organic controls. Here's a link to information about just one (of many) fungi that infect rhododendrons, with some cultural and chemical controls. (The "H" after a chemical means it's available to home gardeners.) You can see that Spectracide Immunox is the only one on the list for this specific plant. Follow all label instructions. Be sure to remove leaves that have the fungus on them, since spores will continue to spread to other plants.
Another method of control is to use an oil, such as neem oil, that is safe on edible plants and, if properly applied, won't harm bees, butterflies and moths, for example. There are other oils available, but neem oil is also effective on black spot (on roses, in the spring) and aphids and other soft insects. You can buy it concentrated (70%) or pre-mixed. Spray your plants early in the morning, when bees, etc., aren't out. Saturate the surfaces of the plant with mildew (might even find some under the leaves). The oil 'suffocates' the fungus and any insects that are sprayed, but once it is dry, it has no ill effects. (You probably should rinse it off before eating the plant tissue, though.) As with chemicals, follow all label instructions!
Hope this is helpful. Good luck!