Answer: What you describe sounds like iris borer damage. Iris borers go through several stages of life. A small, nondescript moth lays eggs on iris rhizomes in late fall. The eggs over-winter in any iris foliage or other debris that remains on top of the rhizomes during winter. In spring, eggs hatch and tiny caterpillars crawl up the emerging foliage and bore into the leaves. Once in the tender young leaves, borers travel between the layers of the leaf, eating their way down to the rhizome. After feeding on the rhizomes, borers drop off into the soil and pupate until mid to late fall when they emerge as a moth to start the cycle over again.
To control, dig up iris plants that are showing any symptoms (in mid- to late-August). Trim the foliage back to a 3-4 inch fan, and wash the soil off the rhizomes. Look them over carefully. Discard any pieces that are decayed. Examine the firm, fleshy rhizomes for holes where a borer may have dug in. If you cannot see the borer, but suspect it is still in the rhizome, you can either cut away that section with a sharp knife or soak the rhizome in a water and 10% bleach solution. This may take a few minutes to an hour or two. If the borer is still there, it will back out and drown. After soaking the rhizomes or cutting a section away, let them dry thoroughly before replanting. Once you have disposed of any decayed or infested rhizomes, go ahead and replant.
Best wishes with your iris.
Q&A Library Searching Tips