This fungus over-winters on dead leaves and stems, so one way to reduce the problem is to clean up the garden in the fall. This removes infected plant material which reduces the amount of fungal matter in the garden next season.
Dig and replant iris (about 6 weeks after blooming) to keep them from becoming crowded. Close plantings encourage poor air circulation which creates good growing conditions for the fungus. Crowded plants also make it easy for fungi to splash from plant to plant, spreading the disease.
In most cases, good cultural practices (fall cleanup and plant thinning) keep the problem low in our western climate. Fungicides are rarely needed.
