It sounds like powdery mildew. If I remember correctly Daconil should work, but I'd check the products' info. to see what fungus they/it control(s).

Some air movement might help, but be sure not to blow the spores onto the other plants.

It sounds a like a severe outbreak if you have dying leaves. I'd carefully remove them so as to not infect any other plants and dispose of them carefully. Be sure to wash your hands and disinfect your clippers to avoid spread.

Powdery mildew is very infectious and kills dahlias. In the Fall we sometimes get very bad powdery mildew and without spraying, the plants can turn completely white and all leaves that turn white die in a week or so. If you spray after the leaf turns white, the leaf still usually dies but the new leaves will be normal green.
I once had an infection on some cuttings that were inside my house and sprayed them and thought I had cured them. I planted them outside and the powdery mildew infected those plants again and about 10 feet of row near them(in July no less!). I sprayed with a systemic fungicide and controlled it. I use Strike (Triadimefon).

Thanks for the heads up. I appreciate it.