Question: While digging and dividing the tall bearded iris plants in my yard, I’ve noticed some of the roots are soft, slimy, and smell awful. Is this some type of rot or could it be the iris borer? How do I know so I can treat my plants? — J.M.
Answer: Several related issues may be affecting the iris. Closely inspect the rhizomes (the horizontal underground stem of the iris) for the grayish-pink borer. It can be found feeding on the inner part of a rhizome. If the iris has been attacked by borers, it may then get hit with a double dose of bacteria, one that colonizes the borer feeding areas, causing a soft rot, while the other bacteria invades the decaying tissue, causing the foul odor.
Other outward indicators of borers are streaked, spotted foliage that eventually turns yellow to brown, and a reduction in flower output. The overall health of the plant is compromised by the damage caused by the borers.
The life cycle of the iris borer begins as an egg that overwinters on an iris plant or in other plant debris. In early May, the egg hatches and the larva bores into the leaves. It tunnels downward through the leaf to the underground rhizome, eventually moving into the soil to pupate. The adult stage (moth) later emerges and will continue the cycle by laying eggs in late August and September.
If borer damage is suspected while digging and dividing the mass of rhizomes, cut off the affected root sections and dispose of them. Replant sections of the iris that are free of borers.
To manage this pest, during fall cleanup in the iris bed, remove and destroy old iris foliage and any other nearby plant debris. This will help remove any eggs and decrease the incidence of iris borers next season. In spring, insecticides containing spinosad, pyrethrins or permethrin may be applied to help reduce iris borer populations, but it is important to apply when the eggs are just hatching. Apply when the leaves are 4 to 6 inches in height. It may be necessary to repeat the application 10 to 14 days after the initial application. Read and follow label directions when applying an insecticide.
Also, parasitic nematodes, Heterorhabditis or Steinernema, contained in soil drench products, have shown success in controlling iris borer larvae when applied in June and early July.
Additional information on iris borer can be located on the University of Wisconsin-Madison Plant Disease Diagnostic Clinic’s link: https://pddc.wisc.edu/2015/07/28/iris-borer/.
Jeanne Hilinske-Christensen is the UW-Extension Interim Horticulture Educator for Kenosha and Racine counties. Submit plant care questions to the Master Gardeners Plant Health Advisers. Phone 262-857-1942 or email firstname.lastname@example.org.