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Picture of the Week
26 July 1999

Iris Borer and Bacterial Soft Rot

Damage by iris borer may cause iris leaves to wilt and discolor suddenly. Soft rot bacteria often enter rhizomes through wounds caused by the feeding of iris borer larvae, resulting in a disease known as bacterial soft rot of iris.

The adult iris borer is a moth. In late August and September, the female moth lays her eggs in clusters on the iris leaves, the base of stalks and other nearby plant debris. The eggs overwinter in the plant material and hatch in April or early May as the new iris leaves are expanding.

The small, young, larvae crawl up onto the new iris leaves and make tiny pinpoint holes as they enter. Once the larvae enter the foliage, they act as leafminers, tunneling to the base of the leaves throughout the spring. This leafmining damage appears as water-soaked, brownish spots and streaks on the leaves. By early to mid-July the larvae reach the soil area and tunnel into the rhizomes.

In the rhizome, the fat-bodied, pink larvae with brown heads grow to be 1 1/2 to 2 inches in length. Their tunneling in the rhizome is particularly damaging to a smaller-rhizomed iris such as Japanese or Siberian iris. A tall, bearded iris, with its larger rhizomes, frequently is able to sustain this damage and still survive.

In late July to early August the iris borer larvae move from the rhizomes into the soil to pupate. Adult brown moths emerge in late August and September to mate, lay eggs and repeat the cycle described above.

Usually the main problem with iris borer infestations is the accompanying invasion of bacterial soft rot. Rhizomes infected with bacterial soft rot become slimy, soft and foul smelling. The combination of bacteria and borer can cause rapidly appearing serious injury.

With close, regular inspection of leaves in from April through June, iris borer larvae can be easily detected at the lowest point of the mine and squashed while leafminers are within the leaf. Dimethoate (Cygon) can also be sprayed on the leaves to kill the larvae as leafminers, ideally when the leaves are three to six inches tall. For better insecticide coverage add a few drops of detergent to the spray.

Fall sanitiation is very important for iris borer control. Following the first hard frost, remove and destroy old iris leaves, stems, rotted rhizomes and nearby plant debris. This will help to get rid of the overwintering egg stage.

Editor's Note: This information appeared in an article by Rosie Lerner and Gail Ruhl in Down the Garden Path Newsletter (#137, October 6, 1998).

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Last updated: 10 October 1999/tlm.