Lyn,Yes I had that problem last year.The only way I noticed were fans lying on the ground.It happened to 6 new iris all in the same area,3 new intro's.The rhizomes mushy.
I cleaned up a few that had nubs attached.2 grew small new iris.
I think it was a wetter than usual summer and fall last year.
No borers,just mushy rot in one area .I have no explanation.
I had not had a rot problem since my first year of iris 30+ years ago.
I might add that there was only a tiny area of the rhizome that I was able to save with a few nubs.Like half a thumbnail.
