The Q&A Archives: Controlling Iris Rot (Iris Borers)

Question: Some of my irises develop soft mushy areas just where an old, wintered over leaf fan stands. I notice that the leaf fan just topples over and is light brown and translucent in appearance. On other plants I can wiggle the fan slightly and it falls over. In the fall I am careful to cut the leaves on an angle and within 6-8" above the ground as advised in gardening books. My rhizomes are planted so that the tops are above the soil level and not mulched. I usually notice this in a February thaw when temperatures start to moderate. Last year we experienced 96" of snow throughout the winter season. This year has been unusually mild with ample rain and the usual cloudy winter days. I have noticed this in the sunny and shady gardens with equal incidents in each so I do not feel location has anything to do with it. I fertilize sparingly with 5-10-5 in the early summer only. Although not all my irises get this condition, it is puzzling that every year some do. What do you think is wrong? Regina Reid N. Guilford, CT

Answer: You may have iris borers. These moth larvae, hatched from eggs laid in old iris stalks and garden debris in late autumn, emerge in the spring as tiny caterpillars. They bore into new iris leaves and eat the soft interior tissue, gradually working theirway down into the roots. The borers not only damage the plants with their chewing but carry the bacteria that produce foul-smelling soft rot, one the serious diseases that affect irises. Clear iris beds of old leaves and stems, where borers lay their eggs in the fall. You may need to spray young plants with a pesticide in early spring, before the borers enter the leaves. Make sure the iris are not receiving too much water..this will also encourage rot.

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