Issue 7, June 5, 2009

Rotting Iris Plants

If you are seeing rotted iris plants, chances are the plants have bacterial soft rot. This bacterium, Erwinia carotovora is present in most soils and will invade through wounds. You have probably seen soft rot on ripe tomatoes that fell before they could be picked, or possibly you have experienced soft rot on your carrots in a plastic bag in the refrigerator. Usually you can smell the problem before you find it. Such is the case with soft rot of iris.

Look for leaves that wilt and die from the tips toward the base. If you pull on the affected leaves, they are often rotted at the base and easily pull from the plant. There is a distinct odor of rotting plant material. Soft rot can enter the rhizomes as well, often leaving only the shell intact.

Several factors could be involved in the soft rot scenario. Wounds caused by iris borers are often the precursor to soft rot. Still, borers are not always the problem. Two years ago, I saw quite a bit of soft rot in my iris bed, possibly as a result of freeze/thaw injury in the early spring before flowers bloomed. There were no borers in my planting. Identify soft rot as the pathogen and then investigate to determine the cause of injury that allowed the bacterium to enter.

In my case the rotted rhizomes were removed, weeds cleared out to provide better air movement, and the iris bed was divided and cleaned in late July. This should be done every 3 to 5 years to reduce the incidence of soft rot. For information on how to divide iris plants, visit this extension link. The rhizome should be just slightly exposed on the soil surface when planted correctly. Deep planting will also cause rhizomes to rot.

Iris borers can be controlled by applying insecticide in April when leaves are 5 to 6 inches tall. It would be wise to inspect rotted plants now for borers so that you can plan to use insecticides next spring. Acephate (Orthene, Ortho Systemic Insect Killer, etc.) or disulfoton (Bayer Advance Garden, Bonide Systemic Granules, etc) will work when sprayed once at this stage of growth. Sprays are no longer beneficial for this year.--Nancy Pataky

Nancy Pataky

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