It is iris fungal leaf spot most likely. It occurs more in rainy or foggy weather. Overhead watering would also contribute. Some years are bad, others none. Doesn't kill the rhizome. Next spring new growth will be clean.

Continue to cut away the infected leaves or trimming. If you wish you can spray now with a fungicide to prevent further spreading. I think directions call for repeat sprays. Daconil is one some people use. I have used whatever was on hand for the roses.

An unsightly thing but not life threatening like soft rot. Digging up the clump is unnecessary. Hopefully next year it won't appear!

Thank you very kindly for the advice iris_gal.
:) Will do, and much appreciated that I don't have to get rid of my "before the storm" clump. I think I've given a dozen people the iris bug with that clump, so I was hoping that I wouldn't have to destroy it.