Someone who used to post on here regularly, and I miss greatly (begoniac) recommended keeping a spray can of Lysol handy for just this kind of problem. Someone else recommended Tinactin. Hope that helps!

I should have said, of course there are sprays just for this kind of problem (fungicides), and it might help to increase air movement, like with a fan.

This sounds like powdery mildew which is common among certain canes and rexes. Cut off the infected leaves. The spray was Lysol disinfectant but other fungicides will help. Air movement is supposed to help. Usually the problem shows up with cooler temps.