This has happened to me for the first time, too! I came to find the same answer ~ lol!
I have bought some Messenger to apply in the hopes that will help...

Okay, so here's what I found online:

If you want to use a non-chemical spray, the American Iris Society recommends the following: To a gallon of water, add three (3) tablespoons baking soda, two (2) tablespoons of shavings from a bar of Ivory soap, and one (1) tablespoon vegetable oil. Apply the spray to infected parts of the plant. This does prevent mildew, and may work on leaf spot as well.
Prevention is the best cure, so if you are in a moisture prone area, to avoid fungal leaf spot it is best to plant irises in full sun, in a rich well drained soil, also try to select disease resistant varieties, and do not water with overhead watering systems.

A few years ago when this was being discussed, people swore by this recipe for fungal leaf spot. I've never tried it, but will pass it along in case someone wants to experiment with this spray:

1 gallon water
1 cup listerine
1 cup lemon-lime soda
1 cup tea

Thanks, Thurston!