Unfortunately, I have to be the bearer of bad news concerning one of our most prized landscape plants: Knockout roses. Knockouts revolutionized the rose industry by giving us a low-maintenance rose that did not need constant spraying for insects and disease, and blooms all season long.
Well, Wednesday of last week, for the first time, someone brought me a cutting of a Knockout rose that had the dreaded rose rosette virus. My fellow agent in Columbia County, Tripp Williams, told me no one had brought the rose problem to him, but a couple of people had described the symptoms to him on the phone.
Just last April a master gardener forwarded me an article about the virus and asked me about it. I told him it sounded like it was only a matter of time before we got it in the Augusta area. In the Atlanta area, retired county agent and gardening guru Walter Reeves received a sample of the problem in November 2010.
Rose rosette virus has been described since the 1940s but it wasn’t until 2011 that the causal agent was confirmed to be a virus spread by the “rose leaf curl” eriophyid mite. Rose rosette virus was found predominantly in multiflora roses that were introduced to the eastern U.S. in the 1800s from Japan. They now grow wild in many places and are considered an invasive/noxious weed. The wild multiflora roses were thought to be how the mite and virus spread into rose landscape plantings.
Knockout roses have been so disease resistant, they blanket commercial and residential landscapes throughout the South. The presence of all these Knockouts provides an easy means for the mite and virus to spread.
Dr. Jean Woodward, the extension plant pathologist, confirms the suspicion that some plants are already infected when they are bought at the garden center.
“If the problem came from the wild roses, it would start at one end and work its way over to the other plants,” she says.
The symptoms of rose rosette virus mimic herbicide injury. They include an increased and rapid elongation of new growth; abnormal reddish discoloration of shoots and foliage; witches broom (proliferation of new shoots); an overabundance of thorns; and deformed buds and flowers.
If you have Knockout roses that you suspect have these problems, control options are few. There is no cure for rose rosette. Roses growing near infected cultivated or wild (multiflora) roses have a high risk of infection. To prevent infection prior to planting, inspect the plants at the garden center for symptoms before you buy them; remove all multiflora roses from the area; and increase plant spacing so rose plants will not touch to reduce mite spread.
There is some discussion in online garden forums and rose breeders that just pruning off symptomatic canes/stems will remove the virus. But according to Woodward, this will not work as the whole plant is already infected. Whether you prune or not and just leave the plant, it will eventually die, and of course, potentially spread to other Knockouts.
If you prune roses that have rose rosette, then prune other roses, you can spread the virus this way. So it is important to clean pruning equipment.
A miticide can help reduce mite (and virus) spread, however, the miticides in retail garden centers are ineffective. An alternative is to hire a commercially licensed landscape professional who can use commercial insecticides that can provide some control.
One last point that Woodward made is that, we might have to look at Knockout roses as a plant that we might have to replace every five years or so or at least until they succumb to this problem. After getting rid of a plant or plants that have rose rosette, you can replace it with a new one. Before replanting, be diligent in raking residue from the infected rose.
REACH SID MULLIS, THE DIRECTOR OF THE UNIVERSITY OF GEORGIA EXTENSION SERVICE OFFICE FOR RICHMOND COUNTY, AT (706) 821-2349 OR SMULLIS@UGA.EDU.