Iris leaf spot
Q: Some of my irises have a problem: their leaves are showing yellow and brown spots. It seems to only attack a couple of iris types, but not others that are right next to them.
A: The yellow flag irises in the back are a different iris species than the others so that’s maybe why they don’t have the leaf spot.
Iris leaf spot is a common malady of iris. The brown spots are inconspicuous until flowers appear. Afterwards they may coalesce and cause the top half of the leaf to die. Though the disease does not attack flowers or roots, it can weaken the plant due to leaf loss. Since the fungus overwinters on old foliage, cut diseased leaves at ground level in October. Dispose of them in the garbage.
If you are a real iris fancier you can spray with a fungicide containing chlorothalonil or mancozeb when the leaves are four to six inches high. Repeat every ten days.
See Iris leaf spot https://www.walterreeves.com/landscaping/iris-leaf-spot/
Leaf spot of iris https://www.pubs.ext.vt.edu/content/dam/pubs_ext_vt_edu/450/450-600/450-600_pdf.pdf