I just went to my side garden today to find one of my peony completely covered in powdery white. From googling around it looks like powdery mildew. I've seen there are various things you can use to forestall, maybe even treat.

My bigger question is this: these are peony that usually get mowed down before winter anyway, so this late in the season, does it make sense to mess around with trying to spray treat the heavily infected plant, or just cut down the stalks, treat the stumps, bag the leaves and solarize them?