Rose rosette virus is spread by a tiny mite, Phyllocoptes fructiphilus, which feeds on sap from tender young rose shoots. The mite itself does little obvious damage to the plant, but if the mite is a carrier of the virus, it infects the rose with the disease. The almost-microscopic mites cannot fly, but they can crawl between neighboring plants. They can also be blown by the wind for hundreds of yards, and it is thought that this method of dispersal has been a major cause of the spread of RRV. The disease can also be spread by root-to-root transmission.
Rose rosette virus is largely hosted by wild populations of the non-native and invasive multiflora rose (Rosa multiflora). This rose was introduced from Japan as a rootstock for ornamental roses and was also widely used in the first half of the twentieth century for erosion anagement. It quickly got out of control and is now a pest plant in many regions of the United States.
Unfortunately, the virus also affects many cultivated rose varieties. Some varieties are especially susceptible, including the widely planted and popular ‘Knock Out’ roses. Since these roses are so ubiquitous, if a stand of them gets infected by RRV, the disease could easily spread throughout the neighborhood.
So what can you do? First learn to identify the symptoms of the disease (and I recommend that you search the Internet for pictures so you will know what to look for). New growth will elongate more rapidly and may be noticeably thicker or thornier than normal. Terminal growth will also be reddish in color. Note that new leaves of many rose cultivars also have a reddish tint, but it fades as the foliage matures. The disease may deform the plant in various ways that mimic herbicide damage. For instance, stems may be excessively branched, displaying a growth pattern called a “witch’s broom,” and deformed leaves may have a strap-like shape and be smaller than usual. Blooms may also be affected, displaying distorted petals that are fewer in number. Symptoms typically appear one to three months after infestation. Once the rose is infected, the virus moves throughout the plant, even down to the roots.
If you do have an infected rose, in most cases it must be dug up. The plant should be fully removed from the ground and then burned or bagged for disposal. The virus is not harbored in the soil, so roses can be replanted in the same area if all roots from the infected plant have been removed. In my research for this article, some sources said that an infected cane may be selectively pruned out if – and only if – the disease is caught while in its very early stages. The mites infect the rose from the tender growth tips, and the RRV then slowly travels down the cane. Therefore, if the damage has just begun and is limited to a cane or two, it may be possible to prune the canes to the base of the plant. Then dispose of them carefully to keep the mites from being spread to the rest of the rose. If you choose to try this method of control, carefully keep an eye on the plant to be sure that you did in fact remove all of the infected parts.
Since treatment options are so limited, rose growers must concentrate on preventive measures, such as controlling the mites via an insecticide labeled for such use. For pesticide recommendations, contact your County Cooperative Extension office.
Other preventive measures include eliminating multiflora roses that are growing near your property and may be harboring the disease or the mites. Make sure that any new roses you plant are disease free, and when planting, to minimize the spread of disease, space the plants far enough apart that they do not touch each other.
If you grow roses, I urge you to inform yourself about the rose rosette virus, monitor your roses diligently, and take proper actions to prevent this deadly disease.
Information about Extension Solutions for Homes and Gardens can be found on the University of Georgia Cooperative Extension website at www.caes.uga.edu/extension/cherokee or by contacting the Cherokee County Extension Office at 100 North St., Suite G21 in Canton at (770) 479-0418. The Georgia Extension Master Gardener Program is a volunteer training program offered through county offices of the University of Georgia Cooperative Extension.