Good sanitation helps with foliar diseases in peonies. As soon as the leaves die back this fall, remove all leafy debris from the area so that spores don't overwinter. This may help next year to reduce this problem.

Also, either enlarging the bed and adding other plants or moving the peony to a bed with other plants may help reduce the impact when it does have a bad year like this since there will be other plants to distract from the peony's messy foliage.

At least some of this is powdery mildew which is more common in years with high humidity and cooler temperatures like this year. It isn't fatal to peonies.

I think that you should cut the foliage and stems down to about 2 inches and dispose of the debris now. You should not wait any longer to do this.

I've had powdery mildew hit my peony for the last three years at about this time of year they would look horrible. I cut them back and threw away the cut off limbs just as kousa says to do. This year they have not experienced any powdery mildew! WOOHOO! They probably will be fine next year.

Powdery mildew is hard to avoid in damp areas but watering at ground level will help, try to avoid wetting the leaves.
We normally don't have an issue with it in CO but we've had a really wet spring and summer, actually had to cut off a bit this year here, too.

Thanks for everyone's help - I'll definitely give this a try tomorrow.

Kousa, just to clarify, do I want to cut ALL stems and foliage down to two inches from the base of the plant? In other words, this should resemble a bare plant when I'm finished cutting everything back, right?

After cutting it down to two inches - it should resemble a cluster of bare 2" sticks above the ground. Be sure to clean up and dispose of all the leaves and debris so as not to allow any rain to splash up and reinfect the plant all over again next season. The spores can winter over in fallen debris.

Thank you - that's what I ended up doing last night. I'll post a picture late to confirm I did it properly.

Regarding the debris, I removed all the preexisting leaves as well as anything that fell from cutting the peony, but we have a very large pin oak tree above the peony, and it sheds just about year round. You can see some of this in the initial picture I attached. Do I need to be vigilant in collecting all the droppings (beyond just the leaves, which I typically pick up), or are these harmless to the peony?

Thanks again for everyone's help.

Duluth is right in his/her clarification of the 2 inches. I think the oak leaves are ok as long as you clear them off and away from the peonies before growth starts, maybe early March.

I've attached a picture of what it now looks like.

I don't need to worry about removing the acorns falling from the oak tree, do I? I sure hope not because that job would have to be done multiple times per day. :) Thanks again for everyone's help.

Personally, I would spray the ground heavily now with fungicide, and again in the spring when it begins to green up. What is it mulched with? I might change the mulch too.

Thank you - is there any particular fungicide you would recommend?

As far as the mulch, we put that on there this year (a couple months ago), and I never thought about that, but maybe that's partially (or fully) to blame. It's inexpensive brown mulch from one of the big box stores; last year, we did not have any mulch down, and we did not have this problem. However, that could be a red herring, as I can't remember if the plant was having these issues prior to putting down the mulch.

For a fungicide, will Spectracide 62325 Immunox 3-in-1 Insect and Disease Control for Plants Plus Fertilizer Spray do the job, or do I need to purchase Spectracide's (or a similar company's) concentration that you mix with water instead of the ready to use spray?

From the photo, I would guess that your peony was infected by Botrytis. In the future, when pruning diseased material, it is advisable to use secateurs disinfected in a 10 percent bleach solution, dipping between each cut. In the spring, remove any diseased material as soon as you see it, disinfecting between cuts. Diseased material shouldn't be composted. Then spray with a fungicide. Good air circulation around the plant helps as does planting in well drained soil or on a berm. Good luck.

I am also having an issue with what I think is a white powdery mildew on my peonies. I read that a mixture of baking soda, canola oil, and dish soap (in water) would work. Would this actually work? Or would you recommend the pruning over the mixture?

Also, will it take long for the peony to get back to size and blooming? If the disease is gone, could it potentially flower next year if I prune it down to 2"? I'm not familiar with peonies.

This late in the season there's no reason to try to keep the leaves--they'll soon die back to the ground anyway. You should just go ahead and cut off all of the mildewed material off, you could even just go ahead and cut back all of the foliage down to an inch or 2 of stem. Make sure none of the bad foliage is left on the ground, get rid of it all!

Peonies do best when watered at ground level, especially if you're in a humid area. Drip lines work well or just aim the hose at the ground instead of leaves when watering, especially if you water in the evening where the moisture will stay for longer. Peonies are actually fairly zeric. I'm in Colorado, considered high desert, and I only water my peonies once a week or so in the summer. I do have the beds pretty heavily mulched, though.

Thanks!

Please read Teaming with Microbes .
Please don't use toxins in your garden such as fungicides.
You are apt to kill everything the plants need.

If you can make it or buy it, probably true vermicompost tea will prevent powdery mildew on next year's leaves.
It's a fair amount of work, but the merit accumulates, in contrast to non organic methods, which require more and more whatever as the soil life is destroyed.

Here is a link that might be useful: Lowenfels & Lewis website