I finally figured out that scorch is the name of the condition that is causing a die off of some of my iris plants. The first sign seems to be a brown dead leaf in the middle of a fan, and when you dig up the plant in an advanced state of scorch, you find that the roots have dried up although the rhizome is still hard and looks fine. I have seen this other years, but didn't really know what it was.
I had some irises that bloomed gloriously this year, and then they just quit growing. I thought it was just the drought so I have watered them a couple times this summer, but it didn't help the ones with scorch. When I dug them it smelled a bit like mushrooms? So I am wondering if it is some kind of fungus maybe. For now I am digging up the ones with this condition and cutting out any of the parts that are still good. New roots will form at the growing edge of a rhizome. So I am sort of experimenting with the sickies to see if there is anything to arrest the disease. Well at least it has cut back on my need for new iris beds!