It is rather difficult to actually kill powdery mildew.....but it is relatively easy to prevent or control its spread, although generally best before the disease takes a firm hold.

Good cultural conditions are your best defense - plenty of light, good air circulation. keeping the root zone moist. Various fungicides and home remedies can help to prevent or reduce the spread of the disease - baking soda, milk, dilute acid preparations (affects leaf pH and halts spore development).

Potassium bicarbonate can actually kill off the spores and eliminate the disease....at least for the remainder of the season, if applied consistently. But PM is very seasonal in nature and will likely be back next year unless you spray preemptively. The good new is that it will rarely ever cause permanent damage to plants and is more cosmetic than harmful

If the PM on your peony is bothering you, you can simply cut back the foliage now instead of waiting until later.

where are you ... big city name ... and describe this summers weather ... dorught.. or too much rain???

what kind of peony ... and do they go dormant to the ground ...

peony can go dormant early in bad years ... and once they start shutting down ... they can get real ugly... and there is nothing need be done when they do ...

in a few weeks... you can probably just cut them to stubs.. and call it a season ... [i leave the stubs.. just so i know where to look for them in spring ....]

if its not that kind of peony... you should have indicated such ...

it never hurts to add a pic ...

ken