Clipping the affected foliage back is what I did to begin with.
First, the scape yellowed and died, then it started on the foliage,
yellowing from the top down and in the center. This was a nice
clump that had increased very well from last year.
I checked for vole or mole tunnels. Wasn't that. The crown was
firm and no indication of rot nor any odor. I sniffed every inch of it
when I dug it up. I also gently spread the fans to look for any type
of insect or disease. Nothing. This has me completely puzzled.
It looked like yours Jan to begin with. Now it looks like this. As
you can see, the middle is still yellowing from the top down.
The outer foliage on the fans have grown taller from where they
were clipped. So at least there is a sign of active growth.