A troublesome leaf spot disease on iris is caused by a fungus, Didymellina macrospora . Usually this disease is more common toward the end of the season, on the upper portion of the leaves. It is not as severe with normally dry weather. If the plants are crowded, shaded and the weather is rainy this fungus can cause spots earlier in the season. This results in spots occurring over more of the leaf surface.
Spots are dark brown at first and surrounded by a water soaked and then yellowing region.
In addition to leaves the flower buds and stems may show symptoms. Control is possible by removing and destroying all old leaves at the end of the season. Also remove and destroy any infected leaves during the growing season. Iris rust caused by the fungus Puccinia iridis can occasionally be troublesome. Symptoms include small, oblong to oval, red or dark brown powdery spots, often surrounded by a yellow margin.