It's my understanding that the fungus is in the soil. When it rains, it splashes soil -- and the fungus -- onto the Iris leaves. It shows up as leaf-spot. I hate the way that it looks -- but I've seen it in photographs of professionals' Iris beds -- so I don't feel so bad when I see it in my own beds. As long as it doesn't harm the Irises, I'll be content to cut away the affected leaves -- IF I have the time/energy/etc.
