What you are describing is probably fungal leaf spot. The fungal spores are in the soil and get splashed up onto the leaves during rainfall or overhead watering. Hot, rainy weather can cause leaf spot to run rampant through an iris bed. Fortunately, leaf spot is not fatal to irises, though heavy infections can weaken the plants somewhat. Unfortunately, leaf spot is very unsightly.

There are several things you can do to avoid or minimize leaf spot:

1) Avoid overcrowding of irises. When infected leaves are blown against healthy ones, the healthy ones can become infected. Proper spacing, excellent air circulation, and full sun exposure can all minimize the spread of leaf spot. Digging and dividing your irises after this bloom season will allow you to thin and space out your planting to improve leaf health.

2) Avoid overhead watering.

3) Keep plants strong and healthy by providing optimal growing conditions and appropriate fertilizer.

4) Remove infected leaves or parts of leaves whenever spotting appears. Trash or burn the cleanings. Do NOT compost infected leaves.

5) If you are not opposed to chemical use in your garden, spray the iris leaves with alternating fungicides every 10-14 days, starting before the first leaf spots appear and continuing for the remainder of your rainy season.

Laurie

Thanks for your reply. You mention to alternate fungicides, what kind would I alternate? Thanks again.

I don't use fungicides in my garden, and the only trade name I can remember is Daconil. I'm sure you can find others in any garden center, though.