Iris root rot?

Asked August 31, 2020, 10:46 AM EDT

My iris are developing some awful root rot. The bulbs turn to yellow mush and stink horribly. What is this and what shall I do? Worried it will also infect other flowers, etc.

Montgomery County Maryland

1 Response

This sounds like bacterial soft rot of the rhizomes; in Irises it is almost always caused by damage from the Iris Borer, a pest which begins feeding inside the leaf in spring and which gradually burrows down into the rhizome itself, consuming its tissues. This injury is then infected by opportunistic bacteria, which cause the smelly and mushy symptoms. Wounds are the predominant way bacteria are able to enter plant tissues, and the spores in soil should not be a danger to any other plantings. No treatment of the soil is necessary (or even practical), and for the Iris, you can cut out any rot, let the injury callous-over for a day or two in the sun, and re-plant. Remove all dead Iris debris over the winter, as this will minimize how many borer eggs survive. If you see leaf damage in spring, remove it, treat, or squish small mines before the borers can make their way back down into the rhizome.