Called "the rainbow flower," irises (Iris spp.) come in almost every color but black, often combining a number of colors in one flower. The most widely grown type of iris is bearded iris (Iris x germanica), which grows in U.S. Department of Agriculture plant hardiness zones 3 through 10, with thousands of cultivars available. The more than 200 different iris species include Siberian iris (Iris sibirica), which grows in USDA zones 3 through 9 and Japanese iris (Iris ensata), which grows in USDA zones 4 through 9. Ordinarily problem-free, sometimes iris develop discolored leaves or flowers.
Fungal diseases can start in iris leaves, crowns or rhizomes, which are the enlarged rootlike stems from which the plants grow. The fungi produce threadlike growths that feed on the plant tissue and then produce spores, which get carried to other plants to start new infections. Leaf spot shows first as small, brown spots with water-soaked edges. The spots enlarge into reddish-brown areas and leaves can die. Crown rot starts near ground level where leaves emerge from the rhizome. The leaves develop yellow tips and die slowly from the tip downward, while white fungal threads appear at the leaf bases.
Prevent Fungal Diseases
Keeping the garden clean is important in disease prevention, as well as keeping all gardening tools, your hands and your gloves clean. Dip tools in a solution of 1 part household bleach and 9 parts water. Wash your hands and gloves before and after handling diseased plants. For bacterial and fungal leaf spots, remove and destroy infected leaves and clean up old iris leaves and flower stalks and destroy them each fall. This eliminates sources of spores. For crown rot, remove and destroy infected tissues and expose the cleaned area to sunlight. Before you plant new rhizomes, dip them in a solution of 1 part household bleach to 9 parts water.
Bacterial crown rot attacks iris rhizomes mostly in moist, warm weather. Whole leaf fans turn brown and die. Remove the soft, unpleasant-smelling rotted areas with a spoon and dust the wound with a household cleanser containing bleach. Leave the wound open to sunlight. Use the bleach solution to disinfect all tools and objects that contact the infected plants. For bacterial leaf blight, which resembles fungal leaf spot, except for the lack of microscopic spores at the spot centers, remove and destroy all infected leaves, again disinfecting tools and observing good garden sanitation measures.
A mild infection of mosaic virus, which is transmitted by aphids, causes yellowish-green stippling on leaves and flower stalks, and severe infections can stunt plants. The iris flowers can show broken color and petals can pucker. There's no cure, and most iris tolerate mild infections. Dig up and destroy severely infected plants.
Iris leaves can become discolored due to nutrient deficiencies. Lack of nitrogen can cause yellow-green, small leaves. Phosphorus deficiency can cause leaves to turn red or purple. It's best to test the soil for deficiencies to confirm which nutrients are lacking before applying fertilizer. Iris leaves can also turn whitish-brown if they get sunburned. Usually this happens when a plant is suddenly moved from a more shaded area into hot, full sun. The plant usually recovers and sends up new leaves.