The bearded iris is a low-maintenance perennial that produces flowers with six petals. Three of the petals are upright and three hang down, forming almost an hourglass shape to the bloom. A fuzzy line, or beard, runs down the middle of each of the hanging petals. The flowers come in pink, purple, blue, white, yellow, red or even bi-colored. Most cultivars flower in April to June, but there are some that flower again in the summer, and again in the fall, including the miniature dwarf, the standard dwarf and the border iris.
Crown Rot Fungus
Crown rot fungus causes the base of the plant's leaves to rot where they join the rhizome, the underground stem that produces new shoots. If you see signs of crown rot fungus (reddish-brown "mustard seeds") trim the leaves to allow for more sunlight and air circulation, then remove any infected leaves.
Mosaic is a viral disease. The disease causes the flowers and leaves to mottle. This disease is transmitted via aphids as they fly from one plant to the next. To control mosaic, control the aphids, which can be killed with a mixture of 1 part dish soap and 5 parts water (Dawn is the detergent most recommended by experienced gardeners). At the same time, remove all infected plants, and make certain to remove the entire root ball.
Bacterial Soft Rot
Bacterial soft rot is one of the more serious diseases that affects the bearded iris. The bacteria enters the plant through an injury, or cuts to the rhizomes. This disease causes the rhizomes to become mushy and to smell rotten. Using too much nitrogen (as fertilizer or in compost), along with poor drainage, promotes bacterial soft rot. The only way to cure the disease is to remove the infected plant. If the bacterial soft rot did not affect the entire rhizome, you may be able to cut off the affected portion, and re-plant the remaining part of the rhizome.
Leaf spot is a disease that causes small, brown spots to form on the leaves of the iris. It is caused either by a bacteria or fungus. The edges of the spots often have a yellow tinge. It is spread by rain and insects, and even by people who touch an infected plant and then transfer the disease to a healthy one. The disease organisms overwinter on old foliage, so prune the bearded iris heavily in the spring by cutting off infected leaves and discard them. Water the plant from below to control the spread of leaf spot disease. Spray the bearded iris with fungicide in areas with high humidity or severe rainy seasons.