Extension Educator, Horticulture
Is it a dream come true? A stealth virus-like organism moves from weed to weed inside a tiny mite to dismantle an enemy weed of our forests. In the last decade or so a disease called rose rosette has been reducing the populations of the dreaded multiflora rose. Designated as an exotic weed along with Japanese honeysuckle and purple loosestrife under the Illinois Exotic Weed Act multiflora rose invades native areas and chokes out native plant species. The Exotic Weed Act states it is unlawful for any person or agency etc. to buy, sell, offer for sale, distribute or plant seeds, plants or plant parts of exotic weeds.
A disease killing out multiflora rose sounds like good natural control of a pest. However dreams can turn into nightmares as cultivated roses may also become infected with rose rosette. Climbers, hybrid teas, floribundas, miniatures and a number of old fashioned roses have been reported as hosts for this disease.
Rose rosette is believed to be caused by a virus-like organism and it is thought to be spread by a mite so tiny 20 would fit on the head of a pin. As the mite feeds it transmits the disease agent. Of concern to rose growers rose rosette, as well as rose viruses, can be spread on pruning tools and through grafting.
Rose rosette is somewhat easy to diagnose since symptoms are fairly distinct. The new growth of leaves and stems of infected plants appears a deep red. Actually kind of a pretty color like autumn leaves. Hybrid teas typically show a color that is more yellow than red. The leaves may show crinkling and distortion or a color combination of greens, yellows and reds. Rapid stem elongation can be an early symptom in garden roses.
Later the plant produces numerous small succulent shoots growing in different directions giving the stem a "witches' broom" effect. The shoots are usually deep red and much thicker than the original canes. The shoots also have many more thorns. The stems appear "furry" with thorns. Some plants may appear to "grow out of it" for a few months but the symptoms will reappear. The plants usually die within 22 months of infection while serving as a source of infection for neighboring roses. A typhoid rose of sorts.
At the onset of symptoms herbicide drift is often blamed. Some herbicides can cause the witches brooms, distorted growth, and discoloration, but they do not cause the prolific production of thorns. In addition, chemical injury would appear on all the roses or broad-leafed plants in the area, whereas rose rosette is only a disease of roses and usually will show up first in just one rose bush. Investigate the use of herbicides in the area, including products applied nearby, on the lawn around the plants, and to the plants themselves.
Control of rose rosette in cultivated roses takes courage. Currently, infected plants cannot be cured or salvaged. Infected plants should be removed as soon as rose rosette is confirmed. Yes, completely removed. Rose rosette is not picky about attacking that $50 rose you just bought. Dig up and remove all plant parts including roots. This material should not be used in the compost pile. Always purchase healthy looking plants.
Controlling rose rosette through mite control has not been shown to work with any consistency. Research suggests that the critical mite transmission occurs in May and June. If miticides such as bifenthrin and pyrethrin are used as a control option, concentrate efforts in those months.
For more information on Rose Rosette and Rose Viruses, check out UI Extension Report on Plant Diseases "Rose Rosette Disease" http://www.aces.uiuc.edu/vista/abstracts/a666.html and "Virus and Virus-like Diseases of Roses" http://web.aces.uiuc.edu/vista/pdf_pubs/632.pdf