There are many sites with borer information. They eat the rhizomes. The only way to save a rhizome is to cut away any dead material, then do a diluted bleach dip, rub the cut surface with Comet, or sprinkle with sulpher. Let dry in a shaded area at least overnight. Replant. But if you do have borers some kind of treatment is required to prevent another attack.

Pinholes in the bottom of old rhizomes are where roots were attached.

I haven't tried the search function with Houzz but they were many posts with information on borers.

http://www.irisgarden.org/borermain.html

You can prevent borers by cutting back the leaves of the iris. I did it in the fall one year but this year I will do it in the spring. The borers will start eating at the top of your iris leaves. If you do a google image search you should be able to see what this looks like. Iris borers have distinct chew marks. You can cut off the damage part of the leaves and probably find the borer (s). Get rid of the damaged leaves. Don't compost them. Last year I also used beneficial nematodes. These are just some alternatives to using chemicals. The goal is to get them before they get to the rhizome.

The beneficial nematodes are applied as temperatures are at or above about 50 degrees F in the spring. I think it is better to do the trimming of leaves and bed cleanup in the fall, after temperatures are dropping lower. The eggs are laid on leaf tips in late summer and early fall, so if you remove those plant parts, as well as any dead iris foliage in the fall, you'll know there should be no borer eggs in your garden.

yes I think it is better to do it in the fall too! I didn't get a chance to this year. Thanks for the additional info on the nematodes.

If I was unable to treat last year and have lost many bearded irises to borers/rot (nurseries thought some of both) is there any chance of the ones coming up now being healthy? I have been sick since last July and am finally feeling better (though not 100%) so my flowerbeds really suffered. Some parts of my bed are bare now it got so bad. So much for the neighbor calling me "The Iris Lady"...