Rose rosette is a lethal disease of roses that may look very much like chemical injury. Unfortunately, there is no lab test for rose rosette, so understanding symptom expression is helpful in deciding whether to remove a plant or change chemical application practices in the area.
Symptoms of rose rosette disease include thick, redder-than-normal stems with many times the normal number of thorns. Multiple stems at the ends of branches produce a witches’ broom growth and often small, distorted, and chlorotic leaves. Some herbicides may cause the witches’ brooms, distorted growth, and discoloration, but they do not cause the prolific production of thorns. In addition, chemical injury should appear on all the roses or broad-leafed plants in the area. Investigate the use of herbicides in the area, including products applied nearby, on the lawn around the plants, and to the plants themselves. Rose rosette disease often appears in spurts, possibly related to increases in population of the eriophyid mite vector.
Rose rosette is caused by a double-stranded RNA, which means that it is a virus-like disease. It cannot be cultured in a lab, and diagnosis relies on symptom expression. Plants usually die within about 22 months of infection. Multiflora, climbers, hybrid teas, floribundas, miniatures, and a number of old variety roses have been infected with rose rosette. Hybrid teas typically show a color that is more yellow than red. So far, no other host besides rose has been found. Our clinic has seen a few cases of this disease on hybrid roses in the past few years.
The vector of this disease is an eriophyid mite, a mite so small that 20 could fit on a pinhead. Eriophyid mites are much smaller than the red spider mites, which are commonly seen on plants. You can see eriophyid mites with a magnifying glass 10X or greater. In the lab we use a dissecting microscope to view the new growth. As we pick apart the buds, the mites can be found scurrying away from the light and heat. Grafting can also spread rose rosette disease.
Currently, infected plants cannot be cured or salvaged. Plants with symptoms should be dug up and destroyed, including roots, when first noticed. It is strongly suggested that multiflora and garden roses be separated from each other as far as possible. The efficacy of mite control has been questioned regarding rose rosette, but if miticides are used, research suggests that the critical mite transmission time is May and June, so concentrate your efforts then. For details of this disease, consult RPD No. 666, Rose Rosette Disease. This can be viewed on the University of Illinois Extension VISTA web site at http://www.ag.uiuc.edu/%7Evista/horticul.htm or it can be obtained from your local Illinois extension office.