Answer: I think the weed killer application is coincidental. The major insect problem of iris is the iris borer, a plump, pinkish caterpillar that enters the plant via the foliage. It moves down into the rhizome and feeds there. Insect-infested rhizomes are usually infected by a secondary rot that leaves them soft and mushy. The best way to control the borers is to remove iris foliage in the fall. Adults lay their eggs on foliage in late summer, so removing the foliage removes the eggs, too.
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