I'm a little "late to the party", here, but I DO spray for leaf spot. As to the "product'....Daconil is one that I use, and there is one by Fertilome, that's "systemic", and one that's "broad spectrum" (has three different active ingredients), and I also use the "garden sulpher" (powder, mixes into water to create a "sprayable" ) if you've been counting...that's a total of four different fungicides. I *suggest* that you use at LEAST two different products, as fungi...as with bacteria...an *immunity* CAN develop, and you end up with "super spot" !
I spray (with a different product, each time, in rotation) on a 10 day-2 week schedule. As was mentioned , iris leaves are "glaucus" (waxy) and to help prevent run off. be sure to use a "sticker-spreader", added to the solution.....you can find it in the garden center, OR, you can use liquid dish soap , about 1 cap full to a gallon of water (and, if you use "anti bacterial" dish soap, that *might* Help with soft rot, too). Another thing that seems to help when spraying the fungicides (or anything else in the garden, for that matter!) is to spray when the plants are "clean"....i.e. after a rain, or early in the morning, when they are dew covered....the product will "spread out" , and" cling to the leaf' much better.
Begin spraying fungicide, as soon as new growth is 4"-6" tall (for the iris), and keep it up until late summer, or early fall ( I also use the same program for the peonies, and the garden phlox). After the spring bloom is done on the iris, I add, to that next spray product (whichever it happens to be in the rotation) Miracle Grow, mixed in at about 1/2 -- 3/4 the recommended rate per gallon. What this does , is cause a "rush" of new leaf growth....the plants will tend to "push off" all the old , tattered leaves, over the space of a couple weeks....makes them look a lot better, and gets rid of a lot of "spore" from the leaf spots that didn't get taken care of by the fungicides.
All that said.....the *best* two things you can do for fungal leaf spot(as well as several OTHER disease problems) is a reall good "clean up" at the end of the season....get all the 'trashy leaves "out of the garden...and into the trash can,NOT the compost pile! and secondly, allow enough "space" between plants to allow good air flow, and sunlight penetration.......the drier the leaves, the less fungus (and helps with soft rot, too !).
But, as was pointed out, it REALLY doesn't hurt the plants, all that much...they are hardy, and do quite well, even with less leaf area for photosysthesis . Some folks cut back the leaf fans...totally....after bloom is done, and, while the new, emerging growth DOES look better....I think a leaf that is 1/2 covered with leaf spot STILL produces more *energy* for the plant , than NO leaf does ! Plus, if you grow rebloomers....mid summer leaf removal will almost guarantee no rebloom !
