If iris beds are neglected, or aren't maintained enough, they're most likely to get leaf spot.
When I would help house-flip, there would be times I'd see neglected iris beds full of tall grass weeds etc. that would be severely browning and have leaf spot, terribly.
I naturalize my perennials, and sometimes during mid-late summer, if I don't keep up on cleaning-up (maintaining) my iris beds (removing the dead leaves, weeds, etc.) they end up looking something like this
So yeah, mine do occasionally get leaf spot, only if I don't maintain them enough.
