Iris Leaf Spot

Leaf spots caused by the fungus Didymellina macrospora (Heterosporium iridis) are a common sight on iris. Small, water-soaked lesions develop rather rapidly into 1/2-inch- long spots with brownish purple centers and yellow margins. The spots are usually observed more on the upper half of the leaves, however, if plants are crowded and wet weather prevails, the spotting may appear earlier in the season, covering more of the leaf and causing more damage. Premature leaf death will weaken the rhizomes or bulbs.

The fungus spreads from diseased to healthy leaves during the summer primarily via splashing water. For this reason, infected leaf tissue should be removed as soon as it is observed. In addition, it is important to avoid overhead watering of foliage, to work with plants only in dry weather, to space plants in order to promote good air circulation and to use more resistant species. Since the fungus over-winters on plant debris, it is especially important to remove infected leaf and flower stalks in the fall.

The fungicides registered for use against this disease are preventive sprays and thus perform best when used prior to infection. However, if leaf spotting is severe, you may help to control the spread of this fungus to healthy leaves by spraying with a registered fungicide at prescribed intervals, according to label directions.

--Gail Ruhl, Interim P&PDL Director, Senior Diagnostician, Dept. of Botany and Plant Pathology, Purdue University