What could the problem be? Several clumps look like this. Horribly hot,dry summer last season. Mild winter. Adequate rain this spring. Excellent drainage.

Do you get borer? If not, remove weeds & check plants for rot. You can take up a single clump to begin with & sniff for bad oder. Leaves don't look like leaf spot.

Knock on wood....we don't get bore here so I don't know what it looks like but this certainly looks like soft rot.

looks to me from the picture that the mulch and weeds are causing rot....clean the area up and get everything away from the iris, and try the old trick of Comet or Dial Soap

D

Mulch should not touch rhizomes so as Dee says when bed is treated & less crowded it may clear up.

Check out the rhizomes, as everyone said to see if it's rot. If it's not, is there a chance something laid on them? I've sure had that happen.

actually that looks like part of mine looked a month ago and I found a rabbit was sleeping there, after I chased him away and straightened up the stems they were OK.