If you see leaf spots on your iris plants this time of year it could be a fungus! Leaf spot disease is a very common problem for iris, narcissus, daylily, and gladiolus. These oval shaped spots begin to appear and gradually turn the leaves brown. This problem can be managed by creating better airflow to allow the plants to dry out, removing the leaves, disposing of the leaves properly (don't compost), and possibly using a fungicide. Leaf spot disease is a manageable condition and may not be entirely curable but there's no reason you can't continue to enjoy your irises for years to come!

This PDF file from the University of Illinois has some excellent information about Iris Leaf Spot: https://ipm.illinois.edu/diseases/rpd...

For fungicide agricultural extensions recommend using ones containing either chlorothalonil or myclobutanil. There are several brands available.