I agree you don't need to do anything, powdery mildew is more cosmetic especially late summer.

Don't cut it them since the foliage is still functional and feeds the plant. Like the other person said, destroy diseased plant material at end of growing season.

My peonies are also like yours usually by end of August each year (begining of aug this year tho), although this summer was a lot more humid than previous ones where I live so maybe that played a role, I don't know how the weather was like in your area.

So basically...nothing to worry about, my neighbor also has one that gets the same powdery mildew each year for like the past 2 decades and its still going strong!

This happened to mine this year, so I've googled it a lot and have learned you can only prevent powdery mildew. So next year, before it happens I'm going to spray mine with a recipe of milk and water, a recipe easily found on the internet for various ratios of milk to water. Once you have powdery mildew its you can't stop it the experts say...only prevention.

@Sue Hughes preventing mildew is fine with some home remedy or commercial fungicide, and I would highly recommend that if it usually happens in the spring or early summer when they need to be flowering and looking good.

Spores for powdery mildew are spread by the wind. But in a lot of places especially those with humid summers, etc,. find that with prevention only pushes it back and by late summer or early fall it still occurs and fairly normal in various areas, when it's late its generally only cosmetic though and not going to kill the plant.

Yeah, even here in dry CO some varieties are covered with it by the end of summer. Just make sure to trash the waste when you cut the plants to the ground each fall.

I am also in NJ, my peonies are showing the same thing. It's been a mild Summer, but a really HUMID one and still humid. My Crape Myrtles are having the same issues this year and usually they are trouble-free. Lavender too seems to resent the humid conditions and am having areas of one browning and dying off in the center.