It is caused by the fungus Didymellina macrospora . The spots enlarge, turn brown and dry. The older spots are surrounded by a dark reddish-brown border. Large dead areas may occur when spots enlarge and merge and leaves may be killed prematurely. The disease is most severe in mild, damp weather, especially in sites where air movement is poor and diseased leaf debris has been allowed to accumulate.
Sanitation is important in controlling iris leaf spot.
Remove all diseased leaves and flower stalks in the fall or early spring. Space plants adequately, keep down weeds, and avoid wetting the foliage when watering. Do not work among plants when the foliage is wet. Plant iris in full sun in a rich, well- drained soil. Fungicides are effective where cultural practices fail to check the disease.