The characteristic grass-like stems and colorful blooms of irises can be found in gardens across the United States. Although young iris plants require frequent watering, over-watering established plants often causes outbreaks of fungal diseases. While many of these fungal diseases cause little long-term harm, bacterial diseases, such as bacterial soft rot, may prove deadly.
Fungal Leaf Spot
Didymellina leaf spot is a fungal disease that causes most cases of leaf spot on irises. The spots of this disease feature gray centers surrounded by reddish brown edges. If left untreated, spots will multiply and merge, resulting in die-back of the leaves. Puccinia iridis, commonly known as rust disease, is another fungal disease that produces rusty, red spots on the leaves. Fungal leaf spots typically occur during wet weather or if the plant is over-watered. Cut back and destroy all infected foliage to avoid spreading fungal leaf spot diseases. In severe cases, apply fungicides according to the instructions on the product.
Bacterial Leaf Spot
Bacterial leaf spots may also occur on irises. These spots look similar to the Didymellina leaf spot and may also cause blight of the leaves. There is no chemical treatment for bacterial leaf spot. Remove and properly dispose of all foliage affected by the disease.
Sclerotium crown rot causes the rhizome to turn brown and soft. White fungus appears on the crown of plants infected with crown rot and reddish brown, seed-like fungal spores may appear on the surrounding soil. Crown rot often occurs as a result of over-watering or over-fertilizing, so these activities should be reduce at the first sign of crown rot. Remove and dispose of diseased plants, as well as the surrounding soil. If the soil is not removed, the fungus may infect nearby plants. Application of chemicals may prevent future attacks of this disease, but will not control it once the plant is infected.
Bacterial Soft Rot
Bacterial soft rot enters the plants through wounds caused by the iris borer, a pest that attacks irises. Plants infected with bacterial soft rot produce a foul scent and experience a rotting of the rhizome and leaves. The best treatment for this disease involves removing the infected rhizomes. Dig out the plant and separate the diseased rhizomes from the healthy plant with a sharp knife. Sterilize the knife between cuttings to prevent spread of the disease. Rubbing alcohol or a mixture of 1 part bleach in 10 parts water can be used to sterilize the knife. Replant the healthy portions of the iris, leaving the rhizome slightly above the surface of the soil. If planted too deep, the iris may experience root rot. Applying pesticides will fight iris borers and prevent future outbreaks of bacterial soft rot.