Answer: Based on your description I am not certain what is happening to your iris. There are several foliage diseases that can cause discoloration in spots. Iris borers can also cause foliage symptoms. And the ants, although probably not directly damaging the rhizomes, would have disturbed the soil and could have interfered with their roots. They might also be tending root aphids which could damage the roots...
I would suggest you consult with your local county extension to try to get a more specific diagnosis and based on knowing that, determine how to treat them. If a chemical control is recommended, they will have the most up to date information on what to use and how/when to use it for maximum results.
In the meantime, good sanitation usually helps with iris problems, so be sure to remove the affected foliage and then do a thorough clean up this fall. If your iris are crowded, you might consider culling them and dividing the healthy ones this summer, then replanting in a location with rich, well drained soil and full sun, and good air circulation. Also avoid overhead watering to try to keep the foliage as dry as possible.
I'm sorry I can't be more specific for you long distance. Good luck with your iris.
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