Iris Leaf Spot – Not Too Late for Control
Gail Ruhl; Sr. Plant Disease Diagnostician
If you did not remove the dead, spotted, leaves from your iris last fall you still have time to practice that cultural control measure before new leaves emerge this spring! You can help reduce the severity of iris leaf spot in your flowerbed by removing old, dead leaves.
Iris leaf spot is a fungus disease that attacks the leaves and occasionally the flower stalks and buds of iris. Infection is favored by wet periods during the spring, and emerging leaves eventually show small (1/8- to 1/4-inch diameter) spots. The borders of these spots are reddish, and surrounding tissue first appears water-soaked. Spots enlarge after flowering and may coalesce to cause significant death to leaf tissue. The disease tends to be worse in wet weather and may kill individual leaves. Though the disease will not kill the plant directly, repeated infection can reduce plant vigor so that the iris may die from other stress factors. Spores are spread to nearby plants by wind or splashing water.
Because this fungus overwinters in old leaves, removal and destruction of dead leaves will help with disease management. For plants that had minimal infection the previous year, this may be all that is needed. Plants that were heavily infected last year may benefit from the application of preventative fungicide sprays containing chlorothalonil (such as Daconil) or myclobutanil. Start when leaves first appear in the spring and repeat sprays as directed on the fungicide label. Iris leaves are waxy, so be sure to include a spreader-sticker (such as dish soap) in your spray to ensure good coverage.