The borers are in the rhizomes right now. The only way right now to get rid of the is to dig the iris and cut them out.
The rot you are seeing is likly more from the hot,wet humid conditions. I have dug some iris recently. I have found no borers but lots of rot on the old mother rhizomes.

Mike G

Thanks, Mike,

I know the borers start early in the hosta, I just thought the hosta didn't usually show the effect til later in the summer.

It really wasn't a good iris year for me. Drat all this rain!