Your plants should emerge fine next spring without powdery mildew. It spreads to some but not all plants. The reason you got it so bad this year could be:

drought mixed with humid air. Powdery mildew sets on plants that are often dry at the root but the leaves have humidity on them constantly

crowded foliage or foliage that is just in a sort of wind blocked area so the air doesn't circulate well and the moisture doesn't dry off leaves quickly

shadier than the plant appreciates

I wouldn't do a thing now but wait. Hopefully next spring will bring better growing conditions and your babies will recover and be mildew free!

GGG

Do a search for "powdery mildew + milk" for the answer to your question. A simple, organic solution that works.

Thanks GGG Should I stop spraying now and just let the foilage die back on it's own?

Nandina~ I did do some searching as far as organic treatment with milk and read alot of conflicting results, by the time I noticed the PM it was all over the place and I wanted a sure fix(even though that did not help much either) ;)

I wouldn't bother spraying now. My cone flowers have browned in the frost, so yours probably will soon.

Ironically powdery mildew is worse in dry weather. Regular rain or overhead watering washes the spores off the leaves.

Zinnias are another plant that gets mildew. But it doesn't over winter.

Maybe next year you will be alert for the first signs of mildew and try the milk treatment. You do not know if it will work in your situation until you give it a try. Lots of gardeners report success using it.

nandina- You bet I will! I am a lazy gardener by nature, but I think I will do some preventive milk spraying through the growing season next year!