Certified GKH Gardening Expert
This is tricky. I would, first, check your soil for nutrient content and pH. Here are some articles that will help:
If maintaining proper soil condition doesn't remedy the issue, then you can look for disease. Most can be cured with fungicides, but some will be harder to rid from the area.
This collection of articles will help you with most aspects of iris care and treatment, as well as offer much more information on the genus: