Rose rosette disease, also known as witches’-broom of rose, is caused by a virus that is spread by a very small, eriophyid mite. All hybrid roses are susceptible, but its main host is R. multiflora, which is considered a noxious weed throughout much of the U.S. First discovered in the 1940s in Wyoming, California and Manitoba, the disease continued its spread and is now considered epidemic from the Great Plains to the East Coast.
“In recent years, it’s found its way into public gardens, home gardens, commercial landscapes, and caused a great deal of plant loss and consternation for consumers and the rose industry itself,” says Brent Pemberton, Texas A&M AgriLife Research and Extension Center.
The USDA Specialty Crop Research Institute awarded a $4.6 million grant to fight the disease in September 2014, which was matched by $4.6 million of in-kind and cash industry contributions.
The research has been led by David Byrne, Texas A&M AgriLife Research horticulturist and holder of the Basye Chair of Rose Genetics in College Station. The group working with Byrne was selected from five university and government research centers with a wide geographic distribution, ranging from Texas to Florida, Oklahoma, Delaware and Maryland.
One of the major goals of the project is to determine sources of RRD resistance and use them in breeding efforts.
In his research, presented in August at the American Society for Horticultural Science’s Annual Conference, Byrne showed data from nearly 500 commercial roses tested in participating states. According to his reports, 81 percent had clear symptoms, 11 percent were suspect and 8 percent had no symptoms.
“We know resistance is out there,” Byrne says. “But clearly, the vast majority are susceptible.”
In 2015, the group began three-year replicated trials in Tennessee and Delaware. These trials use augmentation, a process by which RRD-infected plant material is placed in contact with the healthy plants to test for resistance.
“We’re actively trying to get them infected,” Byrne says. “Basically, they take infected shoots with lots of mites and put it on the test material to hopefully get the mite to transfer and pass on the virus.”
This is done two or three times per year. Some of the more susceptible roses showed symptoms in three months. Christian Bedard, research director and licensing manager at Weeks Roses, says one of his company’s roses is primed for a breakthrough after five years of testing in Tennessee, including two years of the augmentation trials.
“We’ve given plants to a professor at the University of Tennessee and he’s been trying to infect the plant with the virus for two years and he hasn’t been successful,” Bedard says. “With all the testing we’ve done, it’s shown to be rose rosette resistant.”
The new rose from the California breeder is called Top Gun and it will be available in 2018. It’s a shrub rose, with red flowers and a burgundy suffusion, hybridized by Tom Carruth from parents Memorial Day and Home Run.
Bedard says disease resistance isn’t enough anymore. While many newer roses are resistant to diseases like powdery mildew and black spot, that doesn’t mean they are safe from viruses like rose rosette.
Mark Windham, distinguished professor of entomology and plant pathology at the University of Tennessee, developed a Rose Rosette Best Management Practices guide as a partnership between American Rose Society and TN AgResearch. With careful scouting and care, roses can continue to be the productive, profitable crop growers and retailers love. But Windham says growers must pay attention to the early symptoms of RRD, remove and destroy symptomatic plants, and keep their site and adjoining sites free from R. multiflora.
“When you get to the point where you have rosettes, which unfortunately is when most landscapers, homeowners, even nursery people see it, it’s too late,” Windham says. “You’ve lost the plant. Mite populations have built up to the point where they are ballooning to other plants. They can’t fly; they just float in air currents. And other plants are likely to be infected.”
Windham also has been running the Tennessee trial site. He’s experimented with different management strategies, including pruning at first symptom detection of RRD, using a green barrier to impede movement of the vector mite and preventive miticide sprays.
Windham found that pruning out symptomatic foliage was ineffective for reducing RRD symptoms from emerging on other canes. While green barriers did not stop RRD, the use of Miscanthus sinensis did slow the disease down. The vector mites’ movement was impeded by the barrier, reducing incidence of RRD in test plots by more than 50 percent.
The miticide trial was more successful. Plants sprayed on 14-day intervals with miticides did not develop symptoms of RRD, and by the end of year three of the experiment, all the plants from the control group had developed RRD symptoms. There was initial concern that the use of miticides could make matters worse by killing spider mites, a potential predator of the vector mite, but that has not become an issue yet.
The researchers still have many questions to answer about the potential of using miticides to stop RRD. Efficacy trials are underway for miticides that weren’t part of the initial trial, and Windham says they’re currently looking at application rate and timing for the miticides that worked.
In addition to the experiments in Tennessee and Delaware to verify which cultivars are resistant, Byrne’s breeding work is continuing, as well.
Crosses have been made among various RRD-resistant rose species and putatively resistant commercial roses on both the diploid and tetraploid level. These populations will allow an accurate assessment of the inheritance of and the identification of markers associated with RRD resistance to accelerate its introgression into a range of rose germplasm.
Byrne is optimistic but pragmatic about the amount of work still on the horizon.
“There is such a wide diversity out there,” he says. “It’s not just a rose; there’s 20 different categories of roses in all sorts of colors; doubles, singles for different parts of the garden. It’s not going to be ‘Oh we’ve got one variety; we’re saved,’ it’s ‘We’ve got one variety, now we can breed and really do it.’”