Last year I dug up & divided a large bed of irises. I know not to plant them too deep & did the best that I could to plant them shallowly, but the rhizomes are not all uncovered - it's difficult to get them to stand up if they are not adhered to the ground. The currently affected corner did well in the spring - the foliage looked great this year & even had several blooms. However, now the base of the foliage of several of the irises in one corner of my iris bed is rotting, such that the whole plant is falling off at once. I dug up a couple of the rhiozmes & they were rotten at the junction of the plant & the rhizome. One of them had many little holes in the bottom of the rhizome, but I have seen these holes in iris rhizomes in the past without the rot (but don't know what they are). I'm attaching photos, including one of the rhizome. What do I do with these irises, and what do I need to know in replacing those that are unsalvageable? Thank you -
Anne Arundel County Maryland
It is likely the Irises suffering rot first succumbed to an insect attack by Iris Borer, and the holes in the rhizomes are probably where the larvae emerged to pupate elsewhere in the soil. These moth larvae first tunnel into the leaves, working their way down into the rhizome and consuming it from the inside. Their damage allows a naturally-occurring soil bacteria to enter the rhizome and start to rot it. (Bacteria usually need plant tissue to be wounded in order to be able to start in infection. Intact rhizomes are much less likely to rot.)
Control of rot thus lies in control of the borers, staring this winter after the top growth has died back. Since the moth eggs overwinter in this debris, clear it off and dispose of it. Next spring and summer, monitor the plants for signs of borers feeding inside the leaf (they will leave visible trails of damage) and crush them when seen; you could cut the leaf off below this point instead, but this removes more functional leaf tissue than necessary. The article linked below goes into more detail about the pests's life cycle, but be aware that its recommendation of imidacloprid insecticide is not applicable here, as this chemical is prohibited for use by homeowners in MD. Instead, trying a product with the active ingredient of spinosad while the borers are still young and in the leaves may be sufficient to kill them as they feed.
In the meantime, you can try excising any rot currently in the rhizomes and letting them "cure" by sitting out to dry for several days before re-planting. Even if the main growth point was lost, other now-dormant growth points (like "eyes" on a potato) may exist on the undamaged part of the rhizome that can sprout next year. No soil treatments are necessary or helpful with regards to bacteria, but the cured cuts will not be as vulnerable to rot.
For overall good plant health going forward, it also helps to make sure the Irises are growing in full sun (at least six hours a day in summer) and soil that drains well, with the exception of the few Iris species that tolerate or benefit from wetter conditions.