Last year I lost most of my irises. The leaves turned brown and the rhizomes were soft and mushy. What caused this and how can I keep it from happening again?
Iris borer is the culprit. This day flying moth lays its eggs in the leaf litter of iris each fall. The eggs overwinter and hatch in spring when the new leaves are about 4 to 6 inches tall. The borer enters the leaf and eats it way through the leaf and into the rhizome where it feeds in early summer. The borer pupates in the soil and in fall emerges as a moth to start the process all over again. Understanding its lifecycle can help with control. Fall cleanup, removing the old iris leaves, may be enough to control this pest. Removing the egg laying site sends the moth looking for a better place to lays its eggs. As you dig and divide iris, remove and destroy any borers that are found. Cut out and discard damaged portions of the rhizome and replant the healthy portions for future blooms. Many gardeners have had success using predacious nematodes to control the borer. These creatures are sprayed onto the plant in spring where they find and parasitize the borer. Check the internet or garden catalogues specializing in biological pest control.