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| '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
Introduction
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
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.
Some symptoms, such as leaf coloration, may be subtle. Although some diseased plants develop very obvious red pigmentation, others exhibit a less striking reddish-pink color on leaf undersides or along the margins of otherwise green leaves. Because the new leaves of many rose cultivars normally have reddish pigments, it may be difficult to determine whether the reddish color is abnormal or not. Therefore, it is important to continue to monitor symptoms on suspect roses. On RRD-infected plants, the reddish color does not go away, whereas on healthy plants, the reddish color usually disappears as the leaf matures.
Witches’ brooms on some diseased plants may be an unusual color of green that can be mistaken for symptoms of a nutrient deficiency. However, nutrient deficiency should affect the whole plant. If these symptoms appear only on parts of the plant, they are probably not due to nutrient deficiency and RRD is more likely.
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A "witches' broom" cluster of small red stems and deformed leaves
from RRD grown on the top of this 'Knockout'.
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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
Symptoms that were undoubtedly due to rose rosette disease were described in the United States as early as 1941. Spread of the disease in the U.S. was linked to the history of the multiflora rose, an exotic plant that was introduced from Japan in 1866 as a rootstock for ornamental roses. During the 1930s through the 1960s, planting multiflora rose was recommended for erosion control, as a bird sanctuary and food source, as a living fence for cattle, for strip mine reclamation, and as a crash barrier on highways. This recommendation ultimately backfired because multiflora rose can produce a million or more seeds per plant and can propagate itself vegetatively as well. It quickly spread and is now declared a noxious weed in several states.
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.
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.
Disease Cycle
Rose rosette disease is most likely caused by the newly named Rose rosette virus, an Emaravirus. In 2011, researchers at the University of Arkansas showed a strong correlation between the presence of symptoms of rose rosette disease and the presence of Rose rosette virus: The virus was present in 84 of 84 rose plants showing symptoms of RRD but not in 30 of 30 asymptomatic plants. Now that the putative virus has been identified, it should be possible to use more rapid tests to confirm Rose rosette virus in plant diagnostic labs. In the past, the disease was diagnosed based on a preponderance of characteristic symptoms or by grafting suspect plant material onto known healthy roses and demonstrating transmission of symptoms after a period of weeks to months.
The disease is transmitted by the eriophyid mite, Phyllocoptes fructiphylus, or by grafting. The wild multiflora rose is very susceptible to the disease and is a common source of inoculum. Cultivated roses planted downwind of infected multiflora rose are especially at risk because the mite vector travels on wind currents from infected to healthy plants. Some growers have observed symptoms on previously healthy plants within four weeks of being planted downwind from diseased multiflora rose.
The causal agent of rose rosette disease is not soil-borne, so it is possible to successfully plant healthy roses in beds where diseased plants have been removed. However, because the pathogen is systemic in infected plants, the virus may persist in RRD-infected root pieces that remain in the soil. If plants regrow from these old root pieces, as multiflora rose is apt to do, they can serve as an inoculum source for healthy plants. Therefore, it is important to thoroughly remove symptomatic plants and ensure that infected plants are not allowed to regrow from old, infected root pieces.
Control
No effective control is available for rose rosette disease in existing infected rose plants, but the disease may be prevented from spreading to healthy plants by using a combination of the following approaches.
Resistance
R. multiflora is the species that appears to be most susceptible to RRD. However, many species and selections of cultivated roses are also highly susceptible, and no cultivars have been proven to be resistant. Although the native species Rosa setigera is reported to be resistant to the disease, one grower has reported increased susceptibility to powdery mildew on plants of R. setigera, which could indicate some level of infection by the RRD pathogen.
A species called the McCartney rose, which exists as a weed in Texas, is susceptible to RRD but resistant to feeding by the mites that transmit the disease. It may be possible, through breeding techniques, to incorporate this mite resistance into cultivated roses in the future. In the meantime, it would be wise to assume that all cultivated roses are potentially susceptible to the disease and to be on the lookout for symptoms of rose rosette.
Cultural Control
Early detection of the disease is the key to effective cultural control. Any suspect roses should be removed and destroyed immediately or monitored for continued symptoms and removed as soon as presence of RRD is ascertained. In some areas, burning is permitted and can be used to destroy diseased plants. If burning is not allowed in the area, plants should be bagged and removed. Diseased plants that have been uprooted should not be allowed to remain in the vicinity of healthy roses because they can continue to serve as a source of inoculum.
If possible, R. multiflora plants — which frequently serve as the source of inoculum — should be eliminated from the immediate vicinity (100-meter radius) of rose nurseries and gardens. Locations where individual multiflora rose plants have been removed should be monitored for regrowth, and any regrowth should be removed and destroyed. Multiflora rose over larger areas is difficult to control and complete removal may not be practical.
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.
Chemical Control
Although there is no compound that will directly control Rose rosette virus, effective control of mites with certain miticides can reduce the risk of spread. Be aware that miticides registered for control of spider mites do not control the eriophyid mites that transmit RRD. Some researchers have obtained reasonable control with either carbaryl or bifenthrin insecticides; however, mites are very small and it can be difficult to get complete coverage. Also, use of carbaryl to control eriophyid mites can lead to outbreaks of spider mites. The insecticide Avid is registered for control of both eriophyid and spider mites on roses.
Use of miticides in the absence of cultural controls is not recommended. One way to use a miticide as an additional tool in a control program is to focus sprays on plants that surround spots where diseased plants have been removed. These are the most likely plants to which mites from within a planting would have moved. Spraying every two weeks from April until September should significantly reduce the mite population and the risk of transmission. Additional sprays may be needed during hot, dry weather when eriophyid mites are most active.
Resources
Amrine, J. W., Jr., and D. F. Hindal. 1988. Rose Rosette: A Fatal Disease of Multiflora Rose. West Virginia University Circular 147. Morgantown: West Virginia University. 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.
Disclaimer: Commercial products are named in this publication for informational purposes only. Virginia Cooperative Extension does not endorse these products and does not intend discrimination against other products that also may be suitable.
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