This disease is caused by a fungus (Didymellina macrospora) that infects only irises and a few other closely related plants. This fungus attacks the leaves, and occasionally the flower stalks and buds. It will not affect iris roots, bulbs, or rhizomes (elongated underground stems). Although the fungus does not directly kill the plant, several years of repeated infection result in premature leaf death each summer, greatly reducing rhizome and bulb vigor. Some varieties of iris suffer leaf dieback even when they are only lightly spotted, while other varieties can be covered with spots before they start to die. When the leaves are wet, or during periods of high humidity, the fungal spots produce spores that are spread to other plants by wind or splashing water. The fungus spends the winters in old infected leaves and debris.
Spray plants with a fungicide containing chlorothalonil, mancozeb, or triadimefon. Repeat the spray every 7 to 10 days until the foliage starts to die back. Clean up and destroy plant debris and clip off diseased foliage in the fall. Spray again when new growth appears, and repeat 4 to 6 more times at intervals of 7 to 10 days. Use a spreader-sticker when spraying (see Applying Pesticides).
Plant Care for Iris