Iris sp. (Iris) may get rust. Rust often causes rectangular brown leaf lesions. There are other leaf diseases (Didymelcina which causes lesions that appear to have greasy margins and Mycosphaerella and bacterial rot). But the more serious problems are the iris borer that allows a bacterial rot inside the leaf and rhizome that the borer carries in on its body. The borer enters the leaf when the leaves are about 4-6 inches high. The insect works its way down the leaf into the rhizome. The bacterium follows. Use a preventative insecticide and sanitation (dig up and remove borers, scrape out decay, dip in 10 percent chlorine bleach solution, dry for several hours in sunlight). Replant so that top of rhizome is visible after watering the rhizomes in. Cut leaves to 4-6 inches in height.
Written by James Schuster, Extension Educator, Horticulture, and reviewed by Bruce Paulsrud, Extension Specialist, Plant Pathology and Phil Nixon, Extension Entomologist, University of Illinois at Urbana-Champaign.