Thank you both for your input. I'm glad it's not leaf spot (and hopefully not something worse!)
I guess we have had a somewhat rapid increase in temperature the past couple of weeks.
The iris (my whole bed of irises actually) was just planted at the end of last summer, so this is their first season. Overall the plants all look great, I've got blooms or bloom stalks on 19 out of 21 of them including this one, so I don't think it's a soil deficiency, but then I haven't had the soil tested so... When I made the bed I mixed some native soil with top soil and garden soil. Fertilized when I planted and again this spring. I'm just hoping whatever it is doesn't spread to all the other plants!
Here is a pic of the whole plant in comparison to some of my others. It's the one closest.
