Here are a couple of possibilities....

Bacterial soft rot. This disease causes the bases of leaf fans to become soft and slimy. The rhizomes will soften and become mushy. This disease is accompanied by a very foul smell. In less severe cases remove the infected fans and scrape away the soft rhizome tissue. Treat the exposed firm tissue with copper or an antibiotic dust to help disinfect this area.

Crown rot is another disease that can affect iris rhizomes. There will be a softening of rhizomes, similar to bacterial soft rot, and small round cream to tan spots on the leaf bases. This disease is not accompanied by the foul smell. Remove and destroy seriously infected plants.

You'll have to dig up the infected plant to check the condition of the rhizome - I wouldn't leave it with the healthy ones. If it looks like either of the above, I might simply discard it as the clumps that are doing fine can be split again in a few years to give you more plants.

Thanks for getting back to me. I thought Siberian Iris was impervious to many of the diseases that affect Bearded Iris.

I dug mine up, and it looked like crown rot. If I'd done it sooner, I might have been about to save at least part of the plant. But almost the entire crown was gone, so I threw it out. What a shame! It had a beautiful root system.