There are many varieties of iris. The main difference between the varieties is whether the plant starts from a bulb or a rhizome. The most common iris bulbs are the Japanese iris and the Dutch iris. Bulb iris will go dormant for part of the year with part of the layered bulb remaining in the ground. Rhizome iris’ have thick root pieces with leaves that come off the end. The bearded iris is an example of the rhizome variety. All varieties can become infected with various diseases
Bacterial Leaf Spot
Bacterial leaf spot is caused by the bacteria Xanthomonas tardicrescens. The spots on the leaves look water-soaked and expand when the weather is wet. However, the spots stop expanding when it is drier outside. To control bacterial leaf spot, it is recommended to remove infected leaves and avoid overhead watering techniques.
Botrytis Rhizome Rot
Botrytis rhizome rot is caused by Botrytis convoluta. This disease causes few leaves to appear in the spring. What leaves do appear will turn yellow, brown, and then die. The infected leaves also develop gray masses of fungal spores and the rhizomes rot. Large glossy black structures called scloerotia form on the infected rhizomes. The only method of control is to destroy all infected plants or plant materials.
Crown rot has symptoms occurring both above and below ground. On the plant, leaves will turn yellow, wither up and die. This starts at the time the irises are in bloom. If the plants are dug up, the rhizomes will be slimy and rancid-smelling. This is known as soft rot. Shriveled, dry and rotted rhizomes are known to have hard rot. Soft rot is caused by bacteria while hard rot is caused by fungi. Control for either type involves spraying weekly with an all-purpose fungicide like Bayer Advanced Disease Control. Removing infected leaves and plants is recommended if infection continues.
Didymellina leaf spot is a fungus that causes oval spots with reddish-brown edges and gray centers. These spots become so abundant that the leaves become too diseased and die. The best method of control is removing and destroying infected leaves. In addition, application of chlorothalonil propiconazole, triadimefon or mancozeb is recommended when the leaves have grown to be 6 to 8 inches long.
Iris mild mosaic or iris severe mosaic can affect flowers and stems. Stunting can occur along with flower discoloring and crumpling. Iris typically survive these viruses, but if the infection is too severe then the plant should be destroyed.