September 2014 Plant Disease Update

Iris leaf spot
Iris leaf spot with water soaked border and fungal sporulation in center of spot

Bearded iris is spectacular in bloom, but at this time of the season, the only thing that may catch the eye are brown and shriveled leaves. The popular German bearded iris is very susceptible to the fungal disease, iris leaf spot (a.k.a. Heterosporium leaf spot).

 Small, brown leaf spots with water-soaked borders appear near leaf tips early in the season. The spots enlarge to an oval shape and are surrounded by a yellow halo. Later the spots turn gray in the center with a dark, reddish brown border. Tufts of grayish brown spores may be seen in the spots.  The spots eventually grow together and cause most or all of the leaf to turn brown. Although the fungus does not attack the underground parts of the plant, early leaf senescence may weaken the plant and cause reduced bloom and gradual decline. 

Iris leaf spot disease image
Leaf spots and blight due to iris leaf spot on bearded iris.

The fungus overwinters on leaf debris, so the most important means of controlling the disease is to remove the old leaves by cutting infected plants back at the end of the season. A registered fungicide can be used to protect new growth in spring, starting when the leaves are 4 to 6 inches high and repeating sprays at 7 to 10 day intervals. The German bearded iris (Iris germanica) is more susceptible to iris leaf spot than non-bearded iris. Siberian iris (Iris siberica) has good resistance to the disease.