Answer: Iris borer caterpillars are the most destructive insect pests of iris. They spend the winter as eggs on old iris leaves and plant debris at the base of iris stalks. In early spring, they hatch into tiny caterpillars and climb up new foliage. They chew pinprick-sized holes and tunnel inside leaves as they continue down toward the rhizome. Their feeding causes streaks that appear tan or water-soaked. Later, the tips of infested iris turn brown and appear to age prematurely. Iris borers do severe damage to iris by consuming the rhizome. Iris borer feeding can allow the entry of a bacterial soft rot. Rhizomes infected by soft rot are slimy, soft and foul-smelling.
There are several iris diseases with similar symptoms that can occur when iris borer is absent, including bacterial soft rot, iris scorch (probably caused by a phytoplasma), and fungal leaf spot (Didymellina macrospora). All of these disease organisms will cause iris leaf tips to brown and iris plants to senesce, mimicking symptoms caused by iris borer.
You may never know what caused your bed of iris to die but I would recommend digging out the plants, rhizomes and all and removing them from the garden. I think I would also avoid planting iris in the bed until you've determined just what may have happened to your plants.
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