This plant disease is caused by a bacterium (Erwinia carotovora). It is a serious and common disease of bearded and other rhizomatous irises. The bacteria enter the plant through wounds in the leaves and rhizomes, which are frequently made by iris borers. As infection develops, the plant tissue decays into a soft, foul-smelling mass. Finally, the plant dies and the inner rhizome tissue disintegrates. Infection and rapid decay are favored by moist, dark conditions. These bacteria live in the soil and in plant debris. They are spread by contaminated plants and rhizomes, soil, insects, and tools.
Remove and destroy all diseased plants; they cannot be cured. Discard diseased rhizomes before planting. If only a small portion of the rhizome is infected, you may possibly save it by cutting off the diseased portion. Avoid wounding the rhizomes when digging them up. After dividing rhizomes, let the wounds heal for a few days before replanting. Plant irises in a sunny, well-drained location. Plant the rhizome shallowly, so the upper portion is exposed. Clean up plant debris in the fall.
This disease is caused by a widespread fungus (Sclerotium rolfsii). It decays and kills the leaf and stem bases, the bulbs, and often part or all of the rhizomes. Crown rot is spread by moving water, diseased transplants, infected soil, and contaminated tools. The fungal pellets can survive for many years in dry soil and during extremes of temperature to reinfect healthy plants when conditions are suitable. Crown rot is most severe in overcrowded plantings during warm temperatures (70°F and up) and moist conditions.
Remove and destroy infected plants, bulbs, and rhizomes and discard the soil immediately surrounding them to 6 inches beyond the diseased area. Plant in well-drained soil with roots covered and rhizomes projecting through the top of the soil. Plant only healthy bulbs and rhizomes. Thin out overcrowded plantings.
Macronoctua onusta The larva of this night-flying moth is the most destructive insect pest of iris. In the fall, an adult moth lays 150 to 200 eggs in old leaf and flower stalks. The eggs hatch in late April or early May. Emerging larvae initially feed on the leaf surface, producing ragged leaf edges and watery feeding scars. They then bore into the inner leaf tissue and gradually mine their way down into the rhizome, on which they feed throughout the summer. The damaged rhizome is very susceptible to bacterial soft rot. The larvae leave the rhizome, pupate in the soil, and emerge as adult moths in the fall.
To kill the borers in lightly infested rhizomes, poke a wire into borer holes. In May and June, squeeze the leaves in the vicinity of feeding damage to kill feeding borers inside. Destroy heavily infested plants and rhizomes. Kill the larvae before they enter the leaves with an insecticide containing malathion. Spray weekly from the time growth first starts until the beginning of June. Clean up and destroy plant debris by April to eliminate overwintering borer eggs.
This disease is caused by a fungus (Mycosphaerella macrospora) that infects only irises and a few other closely related plants. This fungus attacks leaves and occasionally flower stalks and buds. It will not affect iris roots, bulbs, or rhizomes (elongated underground stems). Infection occurs early in the season, but spots do not appear until flowering. 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 winters in old infected leaves and debris.
Spray plants with a fungicide labeled for leaf spot, following label directions. Clean up and destroy plant debris and clip off diseased foliage in the fall.
This plant disease is caused by a virus. Depending upon the species or variety of iris, symptoms of infection can vary from barely noticeable to quite severe. Irises grown from bulbs (such as Dutch iris) are most severely affected. Mosaic rarely causes a plant to die but can weaken or disfigure it extensively. The virus increases in the bulbs and rhizomes year after year. Successive plantings of diseased bulbs and rhizomes provide only poor-quality flowers and foliage. Mosaic is spread by aphids. These insects feed on the diseased plants and transfer the virus to healthy plants at later feedings.
Remove and destroy severely infected, weakened plants. Keep the aphid population under control by spraying infested plants with an insecticide labeled for aphids.
Several closely related fungi (Puccinia species) cause this plant disease. The rust-colored pustules are composed of millions of microscopic spores. Some of the spores spend the winter on iris leaves that have not died back entirely, while others overwinter on other kinds of plants. Infection usually starts in the spring as soon as conditions are favorable for plant growth. Splashing water and wind spread the spores to healthy plants. Because iris varieties vary greatly in their susceptibility to rust, some leaves may be killed prematurely, while others may not be affected. Rust is greatly favored by wet weather.
Water in the morning to allow the foliage a chance to dry out before nightfall. Plant rust-resistant varieties, if available.