The recent release of new hybrid landscape roses, including the Flower Carpet and Knock Out series, has raised interest in these ornamental plants for growers, landscapers and homeowners. According to the USDA-National Agricultural Statistics Service’s Nursery Crops 2006 Summary, deciduous shrubs, including roses, accounted for $648 million in gross sales.
Deciduous shrubs were the second largest contributor to sales — behind broadleaf evergreens with sales of $839 million — and accounted for 14 percent of the nursery sales total. But as breeders continue to develop rose varieties that are more disease resistant and easier to grow, one disease continues to elude their efforts to hybridize resistant varieties: rose rosette disease.
Symptoms Vary Based On Rose Species, Cultivar
Although symptoms of this disease were seen as early as the 1940s, the cause of rose rosette was not reported until 2011. Researchers at the University of Arkansas identified a negative-sense RNA virus that is strongly associated with rose rosette and developed detection tests for the virus. They found it in more than 80 plants, which showed disease symptoms. The virus was detected in all diseased plants but not in healthy-looking plants.
Plants infected by the virus can show a variety of symptoms. The severity of the symptoms differs with rose species and cultivar. The most severe symptoms are witches’ broom, lateral shoot elongation, bright-red coloration on leaves and malformation of flowers and leaves.
Growers should not assume the witches’ broom symptom is an indicator of rose rosette. This symptom can also be associated with certain types of herbicide damage. Fall applications of the herbicide glyphosate that contact the green tissue of rose plants can result in translocation of chemical into the buds. However, herbicide damage symptoms don’t become evident on the plants until the buds expand the following spring. Witches’ brooms symptoms caused by glyphosate injury usually appear as yellow, narrow leaves on clusters of shoots.
Flowers infected with rose rosette may produce fewer petals, and their color may be abnormal, such as a mottling coloration pattern. Flower buds may abort, be deformed or develop with leaf-like tissue. Infected rose canes may display excessive growth with red or green pliable thorns that eventually harden. Diseased canes may develop slowly, grow in a spiral pattern and be thicker than the parent canes from which they developed. Plants with the disease usually die in two to five years depending on the rose species.
A plant may display few of the symptoms, especially in the early stages of the disease. As the symptoms become severe and recognizable, there is an increased likelihood the disease will spread to nearby rose plants.
How The Disease Spreads
The virus associated with rose rosette disease is vectored by a small eriophyid mite (Phyllocoptes fructiphilus). The mite is transported over long distances by wind currents, and the virus can also be transmitted during propagation by grafting. Once a plant is infected with the virus, it becomes systemic. Although the virus is not soil-borne, it can persist in root pieces that remain in the soil. The wild rose species Rosa multiflora is highly susceptible to the disease.
Multiflora rose has become a primary host and source of the virus. Originally, it was introduced into the U.S. in the 1860s as a rootstock for ornamental roses. During the 1930s through the 1960s, the U.S. Soil Conservation Service recommended planting multiflora rose for soil erosion control, wildlife conservation and strip mine reclamation. It was also recommended as a living fence for cattle and a highway crash barrier.
Multiflora rose produces millions of seeds per plant and can also propagate itself vegetatively. It spreads quickly and has become so well established it is considered an invasive plant or noxious weed in some states, including Iowa, Ohio, West Virginia and New Jersey.
Increased reports of rose rosette disease have occurred within the last two years, says Michael Dobres, managing director of NovaFlora LLC, the research division of Star Roses and Plants/Conard-Pyle Co. The disease is not as prevalent beyond the Rocky Mountains because there are less plantings of R. multiflora than in the Midwest and on the East Coast.
“There has been a steady west to east movement of the disease over the last decade,” Dobres says. “If rose rosette continues to become more prevalent, we would hope that R. multiflora would be designated as an invasive plant by more states. Multiflora is around because of its extremely high seed set and because it is invasive. As an industry we should try to discourage the use of multiflora as much as possible.”
The good news is some wild rose species have shown resistance to the disease.
“There are about a half dozen rose species native to the U.S. that appear to have resistance to rose rosette,” Dobres says. “We’re actually looking at these roses at Star Roses and Plants. We’re looking at some of the genes from these roses through cross-pollination into some commercial hybrid roses. From the breeding standpoint, we are looking at disease-resistant wild varieties to develop ornamental hybrids. But developing disease-resistant hybrids is something for the future and is expected to take quite a while.”
Controlling Rose Rosette
Currently there are no chemical controls for the virus that causes rose rosette disease. Carlos Bográn, manager of technical services at OHP Inc., says growers need to develop a relationship with their rose propagators to ensure they are receiving disease-free roses.
“There has to be an agreement between the grower and propagator,” Bográn says. “They have to be partners in minimizing the spread of rose rosette disease. If a grower is not sure whether his plants are infected with the disease, he has to have the plants’ virus indexed like other virus diseases, such as hosta virus X.”
One of the most effective ways of controlling the spread of the disease is to eliminate multiflora rose from the immediate vicinity through cutback and the application of herbicides, Dobres says.
“We have prepared a Rose Rosette Disease Guide that lists various herbicides effective in controlling multiflora,” Dobres says. “The ability to use those herbicides may vary by state, and eliminating multiflora rose in every state is a nearly impossible task. We have to focus on eliminating it from areas surrounding landscape rose plantings and around grower production areas.”
The multiflora rose usually blooms during May and June. Plants produce fragrant, 1-inch diameter, white-to-pink flowers. Small, bright-red rose hips develop during the summer, become leathery and remain on the plant through the winter.
“Good cultural practices are essential and will go a long way in reducing the incidence of this disease,” Dobres says. “The most practical recommendation is for both growers and landscapers to do a heavy prune of dormant ornamental roses during late winter just before new growth appears. If growers and landscapers remove the upper portion of ornamental roses where eriophyid mites can be found in the leaf axils, it will significantly reduce the disease pressure on the plants.”
Controlling The Mites
Controlling the eriophyid mite that vectors rose rosette disease can be an effective deterrent in the spread of the disease, Bográn says.
“Growers don’t have to be concerned that this mite will attack and spread the disease to other ornamental plants,” he says. “They have to be concerned about roses, in particular multiflora.”
Three miticides (Avid, Akari and Judo) along with horticultural oil have been listed as controls in Conard-Pyle’s Rose Rosette Disease Guide.
“We advise growers who use these miticides to rotate between the three chemicals,” Dobres says. “It is important to rotate these miticides so that the mites don’t build up resistance. These are contact controls and it is important to use them in rotation.”
Bográn says growers should also consider testing Kontos, a systemic insecticide/miticide labeled for spider mites and other sucking insects, on greenhouse and nursery crops. It can be used as both a spray and drench application.
“The benefit of using a systemic is that it is active in feeding sites that may not be reached by a spray application,” Bográn says. “Also, since the chemical is systemic and is taken up through the plant roots, a drench provides longer residual activity than a spray application.”
Plants should be monitored on a weekly basis throughout the growing season, which is usually April through June, depending on where a grower is located. Ornamental roses showing symptoms of the disease should be removed and discarded by burning or placing plants in plastic bags.
“A grower should confirm that the symptoms have been caused by the disease and not herbicide damage,” Dobres says. “If a diseased plant is identified, it should be removed and destroyed, both in the production environment and in a landscape planting.”