I have a bearded iris that is not doing well. I have identified it as "Supreme Sultan". The leaves are browning and the growth is stunted. It's situated between two other irises that are unaffected. They were crowding out the sick one so I dug it up and I am in the process of preparing a new bed for it.
Should I treat the rhizomes with something before planting? Should I cut off the brown leaves? Any help would be appreciated.

Without seeing your iris,your iris leaves look dry and the roots look like they are out of the ground.
I think it is in need of dividing and replanting and fertilizing with a low 1st number fertilizer or bone meal.
The top of the rhizome should sit on the surface or with a small amount of dirt on top with the roots covered with dirt.
You can trim the fans back to maybe 5 or 6 inches,not real low.
There are directions on line of how to divide your iris.
Don't forget to water for the first week or two after replanting and as needed after that.

Have you tried looking for a hidden infestation of iris aphids, by pulling apart one of the fans at the base? They tend to hide down in the cervices between leaves.

Thank you for the feedback. I found no aphids, but it was in need of dividing. It's in its new bed now and separated into five new plants and some for the neighbour. Thanks for the advice.