Powdery mildew can only overwinter on plant tissue, not in the soil, so make sure you clean up well in fall and dispose of all affected plant debris. In my area, shasta daisies overwinter with a basal growth of foliage so unless you are very careful, it is possible for the PM spores to remain present. Cutting back now can help to remove currently diseased foliage and will also open up the plant to better air circulation to retard any further development.

btw, powdery mildew on perennials and shrubs is almost entirely cosmetic in its impact and causes no long term damage to the plant. It just looks nasty!!

some years.. its impossible to win ...

if they are blooming ... just leave them ... cut back hard in fall ...

i might not cut back hard now ... for fear of full regrowing ... and wasting energy that they should be storing for next spring ...

on the other hand.. i just might get so fed up with it all ... i might chop it all down.. lol ...

there... both answers... lol...

do you water with a wand.. only wetting the soil??? .. thats another trick to reduce it if you water in the evenings .... you dont want plants going into dank dark wet ...

if you have multiple plants of each.. experiment ... thats how i learned most things.. and its how i actually remember things....

ken