It could be powdery mildew. There are several fungicides on the
market that treat the mildew. I suggest you call your local
cooperative extension agent (check the blue pages in the phone
book , and it might be listed under the name of the state
university in your area that handles the cooperative extension
service) to help you get a good ID of the fungus. This will help
you pick the right tool to save your plants (powdery mildew isn't
usually fatal, but weakens the plant to other infection or pest -
I DON"T KNOW ABOUT YOUR DAHLIAS). It also helps you to avoid
picking a chemical that may be harmful if applied to the wrong
DO NOT GO TO THE Garden CENTER FOR THE ANSWERS TO THESE QUESTIONS!
They sell chemicals that you may or may not need. But they are in
the business of selling Both chemicals and Dahlias.