I have never seen it anywhere else in my garden except in my iris beds. That is not to say for sure that it doesn't exist, but the nature of the iris root system is different than almost anything else.

Be sure to completely dispose of the rotted iris, don't put them in a compost pile, not worth the risk.

There is an iris forum, check with those folks.

Jim

It is probably an iris borer.... grubby white things that turn into moths. They get in through leaves and u might see holes in leaves that turn brown. They cause the rhizome to rot. :( I found one while dividing mine....nasty looking things!