Bearded irises have few problems, but there are a couple common ones that may crop up: iris borer and leaf spot. They don’t kill your plants right away but weaken them over time until they finally die. One practice that keeps bearded irises healthy is getting rid of the foliage in late fall. Cut back to the rhizome or tug away dried or tired-looking leaves and burn, bury or send them away in the trash, especially if you’ve noticed any problems over the growing season.
Iris borer is caused by the iris borer moth (Macronoctua onusta) that lays eggs in leaf litter in fall. When the larvae hatch in spring, they quickly find fresh new iris foliage to eat, gradually working their way down to the rhizome and finally out into the soil to pupate. If you notice brown streaks or collapsed foliage (or both), dig up the rhizome and check for a small caterpillar like the one in the photo above. Pesticides aren’t usually effective here so use fall cleanup to short circuit the borer’s life cycle. After killing the catepillar, try saving the plant by cutting away the rotten parts and dipping it in a solution of one part bleach to four parts water. Let it dry for a couple of days and replant.
If your bearded irises have spotted leaves like those in the photo above, either bacterial or fungal leaf spot is the culprit. They’re hard to tell apart without a test from your extension agency. Both show up during periods of mild, wet weather, but the fungal type continues until temperatures get below freezing. Get rid of infected foliage right away. Follow it down to the rhizomes and cut it off with pruners that have been disinfected with one part bleach to nine parts water. If foliage test results indicate a fungicide will help, spray Daconil in spring when the foliage just emerges to knock out the fungus. When bacterial leaf spot is the culprit, you’ll need to dig out and discard those plants in the trash.
Your spring iris show doesn’t have to be over in a flash. Troubleshoot when needed and you can enjoy the color for weeks.