Can you post any photos? Powdery mildew is seldom encountered with iris.....even here in the almost perpetually damp coolness of the PNW!! But you can encounter other fungal issues, however proper identification is necessary before anyone starts recommending treatment options.

Yes, here are two photos. When I knock the leaves, there are bits of powdery dust that flies about too.

That looks like an insect problem, not a disease. In fact, when I enlarge the photos, I see evidence of eggs, nymphs and excrement. Can't tell for sure what they might be but mealy bugs and aphids are possibilities. Try hosing the plants down with a strong stream of water. If that doesn't clear things up, you can try insecticidal soap or neem oil

ok, I'll try that approach first. Thank you very much!