Monty I would wash it under a stream of water using my fingers to feel for any soft areas and removing them. Then I would let it dry for a day or two before replanting. The bacteria that causes a lot of the common rot is in all garden soil.
When you get a chance pick up a container of Comet. You can find it in all stores that sell any kind of kitchen cleaners. You can sprinkle it on the rhizome before planting.
Remember when using bleach that it kills good as well as bad things in your soil. I have washed rhizomes in a mixture of ten percent bleach to ninety percent water but I do not pour it into my soil.
Every gardener has to find his or her own way and this will be your first skirmish against threats to your irises. Good luck.
