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Rose rosette disease is a lethal disease of rose that has been seen off and on in the Evansville area for many years. Unfortunately, 2011 appears to be an "on" year.

Symptoms of rose rosette disease are highly variable, depending on the species or cultivator of rose affected. Some of the more recognizable symptoms include rapid elongation of new shoots, followed by the development of clusters of small branches (a symptom known as "witches' broom").

Leaves in the witches' broom are small, distorted and often noticeably red in color.

Canes often develop excessive growth of unusually soft and pliable red or green thorns, which may stiffen later. Canes also may be noticeably thicker than the parent cane from which they emerged. Flowers may be distorted with fewer petals than normal, and flower color may be abnormal. Infected rose plants often die within one to two years.

Rose rosette is thought to be caused by a virus or a virus-like pathogen. It is spread by the eriophyid mite, which is the smallest of all mites commonly found in the U.S. (less than 1/100 of an inch long) and virtually impossible to identify in the garden.

The wild multiflora rose is highly susceptible to this disease and, in fact, the virus was used for years as a biological control of this invasive weed.

There are no pesticides that will protect a plant from a virus disease. The miticides (insecticides for mites) registered for control of spider mites do not control the eriophyid mites that transmit rose rosette disease.

While some researchers have found the insecticide carbaryl (Sevin) works against these mites, it has no effect on the more common spider mite. In fact, Sevin kills off the predators that would keep spider mites under control.

Once symptoms are seen in a rose bush, it is safe to assume the entire plant is infected. Therefore, the best course of action is to immediately remove infected plants. Do not put them in the compost pile, or pile them up for later burning/destruction. Bag them and place them in the trash immediately.

Destroy all multiflora roses within 100 yards of your rose garden. These wild roses can serve as a source of inoculum. Although the virus doesn't live in the soil, it can survive in any rose roots left behind. I would recommend replanting the bed with anything other than roses, at least for a few years.

To prevent infection of new transplants, avoid planting cultivated roses on hilltops or downwind of known multiflora rose plantings. Space plants so that canes and leaves don't touch each other. Eriophyid mites don't have wings and must crawl from plant to plant. Proper spacing makes it more difficult for the mites to move within a planting.

For more information on rose disease problems, contact the Purdue Extension Service at 812-435-5287.

Larry Caplan is an extension educator-horticulture with the Purdue University Cooperative Extension Service, Vanderburgh County/Southwest Indiana. You can send email to him at LCaplan@purdue.edu.