I have one bed with 13 dahlias and over half have what I thought was powdery mildew. After looking closer, I'm now sure it's spider mites on the underside of the leaves. I've done some reading the past couple days and it seems there are no insecticides available to gardeners that will completely eliminate them. There are some stronger pesticides but sold "for commercial use only." It sounds like most gardeners just spray the mites and eggs off with a water hose. But I can't imagine that they don't return. I have replaced three plants already and plan to replace more or all when it cools down a little. I hate to transplant anything in the heat of July. I will spray Bayer's Insect and Mite Control as I plant and hope to stop it or at least keep it contained to this one bed. I can vaguely recall when gardening was fun. But I swear, there's a new problem(s) every year!
Erin, do your leaves look like this?