If your problem is the crown rot caused by Sclerotinia sclerotiorum, then a soil drench of Daconil, or drenching the base of the iris clump with Daconil, will give control, but not a cure, so in other words, treatment will stop the pathogen from spreading to healthy plants. Terrachlor is another treatment recommended for Sclerotinia rolfsii, or Mustard Seed Fungus which sometimes strikes iris in more southern states, especially areas with summertime heat and humidity.
It is important to identify, exactly what the pathogen is, because some sclerotinia strains have developed some resistance to fungicides. Maybe your local extension service can help.
