I have been battling fungus in my iris gardens for a couple of years now. It is nasty. But I'm starting to believe I may be making some headway (can't give up on it I have way too many varieties). Here's what I have done to fit the fungus "amung us"...
-clear ALL leave debris around the iris' often.
-cut off any infected leaves as soon as they show a fungus spot on them (cut the leaf to just below the fungus spot)
-spray all sides of the leaf with a fungucide per the directions on the bottle (usually every 7-10 days unless you get a lot of rain)

Usually by the middle of summer once the rains stop and we hit the heat, the fungus will settle down a bit.

I have not lost a plant yet, they just seem to keep producing more and more shoots for me. So they are fighting the good fight along with me :)