What was once one of the easiest perennials to grow, bearded iris is now having to fight off infestation by an insect called an iris borer. It is one of the few diseases or insects to bother this rugged plant, but dealing with it can be a challenge.
The first thing you might notice is streaks and wandering tunnels on your iris leaves that later yellow and brown. Iris that is severely infested may also fail to bloom. Most gardeners only see iris borer when they are digging up damaged plants and looking over the rhizomes. At this stage, they are 1-2 inches long, pink-gray in color, with some dots on their sides and a brown head. Iris borers go through several stages of life. A small, nondescript moth lays eggs on iris rhizomes in late fall. The eggs over winter in any iris foliage and/or other debris that remains on top of the rhizomes during winter, in spring, eggs hatch and tiny caterpillars crawl up emerging foliage and bore into the leaves. Once in the tender young leaves, borers travel between the layers of the leaf, eating their way down to the rhizome. After feeding on the rhizomes, borers drop out into the soil and pupate until mid- to late-fall when they emerge as a moth to start the cycle over again.
If you suspect you have borers in early to mid-August, dig up iris plants that are showing any symptoms. Trim the foliage back to a 3-4 inch fan and wash the soil off the rhizomes. Look them over carefully. Discard any pieces that are decayed. Examine the firm, fleshy rhizomes for holes where a borer may have dug in. If you can’t see the borer but suspect it is still in the rhizome you can either cut away that section with a sharp knife or soak the rhizome in a water and 10% bleach solution. It may take a few minutes or an hour or two, but if the borer is still in there, it will back out and drown. After soaking the rhizomes or cutting a section away, lay them out where they can dry thoroughly before replanting. Once you have disposed of any decayed or infested rhizomes, go ahead and replant.
Treat the rhizomes dust the rhizomes with a powdered fungicide such as sulfur or bulb dust, to avoid any decay on the new breaks or cuts. At this stage in their life, the iris borers are not susceptible to any kind of insecticides. As you work the soil, watch for borers pupating in the soil. Pupa are shiny, red-brown, segmented, about an inch or so long and narrow. Dispose of them.
Black, mushy rhizomes that were filled with little white worms. As disgusting as this might seem, those are maggots and they are the good guys and don’t hurt your iris. Maggots are the clean up crew, going in where nothing else will and making use of the decaying tissue.
There are cultural methods of controlling iris borers. There are several things you can do to minimize your borer problem. In the fall, don’t allow any iris leaves or stalks to remain in the area or on the rhizomes. This is where the eggs will over-winter.
Winter protection There are lots of opinions on this issue. Iris that are not well established before winter need protection, but providing protection may encourage borers. If you planted your iris late or you feel they are in an exposed location where they might be damaged by winter, wait as late as you can and then mulch. You want the ground to have just frozen. It isn’t even too late if we have already had the first snow of the season. Then in spring, as soon as the snow cover is gone, remove the mulch.
While all types of Iris can be damaged, it is almost always on tall bearded iris. Iris with bulbs or rhizomes that are deeper in the ground seldom get borers. Damage to iris, but isn’t the borer can be Slugs they will cause ragged holes in leaves that may yellow and brown. There are a few types of leaf spot that cause water-soaked yellow brown patches. And iris can develop soft rot from too much moisture in poorly drained soil.
Bachman’s © 2010