|'Knockout roses' can be susceptible to the viral|
rose rosette disease (RRD).
Symptoms can include excessive thorniness
and permanently red growth.
By Chuan Hong, Extension Plant Pathologist, Hampton Roads Agricultural Research and Extension Center
Mary Ann Hansen, Extension Plant Pathologist, Plant Pathology, Physiology, and Weed Science, Virginia Tech
Eric Day, Extension Entomologist, Virginia Tech
Rose rosette disease (RRD), a disease believed to be caused by the recently identified Rose rosette virus, has been spreading through much of the wild rose population of the Midwestern, Southern, and Eastern United States for years. It has been confirmed in cultivated roses in Virginia and other states. RRD is of great concern to the nursery industry and to many home gardeners because it is known to be lethal to the wild multiflora rose (Rosa multiflora), and it is potentially lethal to many ornamental rose species and cultivars. It has long been known that the eriophyid mite, Phyllocoptes fructiphilus, spreads the disease, but the likely viral pathogen was only recently identified.
Symptoms of RRD are highly variable, depending on the species or cultivar of rose affected. This variability can complicate diagnosis. Some of the more recognizable symptoms include rapid elongation of new shoots, followed by development of witches’ brooms or clustering of small branches. Leaves in the witches’ broom are small, distorted, and may have a conspicuous red pigmentation, although red pigmentation is not a consistent symptom. Canes on some species or cultivars develop excessive growth of unusually soft and pliable red or green thorns that may stiffen later. When this symptom is present, it is diagnostic for RRD.
Symptomatic canes may also be noticeably thicker than the parent cane from which they emerged, or they may grow in a spiral pattern. Flowers may be distorted with fewer petals than normal, and flower color may be abnormal. For example, flowers that are normally a solid color may be mottled. Buds may abort, be deformed, or be converted to leaf-like tissue. Infected rose plants often die within one to two years.
When all of the above symptoms are present, diagnosis is relatively straightforward. However, a diseased plant may exhibit few of these symptoms, especially in the early stages of the disease. By the time symptoms are severe and recognizable, the disease is likely to have already spread to neighboring plants.
A "witches' broom" cluster of small red stems and deformed leaves
from RRD grown on the top of this 'Knockout'.
Other symptoms of rose rosette disease that may be expressed include:
- Blackening and death of the canes on some cultivars.
- Short internodal distances.
- Blind shoots (shoots that do not produce a flower) that remain blind.
- Greater sensitivity of reddish purple tissue to frost.
- Roughened, "pebbly" texture to leaves.
- Increased susceptibility to the fungal disease, powdery mildew. This is especially evident when nearby roses known to be highly susceptible to powdery mildew do not develop signs of this disease.
History of Rose Rosette Disease
Multiflora rose is highly susceptible to RRD — so much so that the disease was initially considered a potential biological control for multiflora rose. Even now, some people suggest introducing RRD-infected plants into areas with multiflora rose as a way to reduce spread of this invasive plant. Most rose growers, however, are very wary of this recommendation because RRD can spread quickly from multiflora rose to cultivated roses.
To prevent infection of new transplants, avoid planting cultivated roses on hilltops or downwind of known multiflora rose plantings where the cultivated rose transplants are more susceptible to invasion by the mites. Space plants so that canes and leaves do not touch each other. Eriophyid mites do not have wings and must crawl from plant to plant. Proper spacing makes it more difficult for the mites to move within a planting.
Amrine, J. W., and S. Zhao. 1998. "Research on Aerial Dispersal of Phyllocoptes fructiphilus (Acari: Eriophyidae), Vector of Rose Rosette Disease." American Rose, March 1998, 28-29.
Laney, A. G., K. E. Keller, R. R. Martin, and I. E. Tzanetakis. 2011. "A Discovery 70 Years in the Making: Characterization of the Rose Rosette Virus." Journal of General Virology 92:1727-32.
Peck, A. 2007. Rose Rosette: A Web Book. Updated May 2007. www.rosegeeks.com.