Remember my catnip? In my lush, lovely windowbox?
Well I whacked it, mob style. Do you want to know why? Powdery mildew! My first encounter. That's when I started noticing, all over the north side of Chicago...
Powdery Mildew! (Edgewater)
Powdery Mildew! (Loyola)
Powdery Mildew! (Rogers Park)
Powdery Mildew! (Also Rogers Park)
Houseplants rarely get powdery mildew because indoor conditions are too dry. However, plants on a patio vacation for summer may contract powdery mildew through airborne spores. The treatment indoors or out is basically the same... remove affected leaves, apply fungicide if needed. Unfortunately, fungicides which are sold for this purpose (and often loudly tout their safety with indoor plants) are often much too harsh. Ferns in particular are sensitive to most chemical treatments, even dilute soap sprays. Always use chemicals as a last resort, use sparingly, allow free drainage of the chemical (no cachepots) and test a small section first. Or you could be like me, and deep-six the whole mess.
As for all the powdery mildew around town, I have no idea. I guess that many outdoor plants can contract powdery mildew yearly in spring and summer without sustaining too much damage, but it seems like it's everywhere, and destroying a lot of the spring flushes on neighborhood trees. It's even in the trees right outside the windows of my plant room, so I'm a little concerned about it entering the compound when the windows are open. We'll see.