Rose Rosette Disease
Rose Rosette Disease
Rose rosette disease (RRD), despite its rather charming name, is a devastating affliction of both wild and cultivated roses. There's no known cure but scouting and management protocols can keep you one step ahead of the blight.
RRD can be tricky. Its transmission involves three players: a virus, which is vectored by an eriophyid mite (Phyllocoptes fructiphilus), and the wild and sometimes invasive multiflora rose. For RRD to be successful, those three elements must be present. To break the cycle of the disease, eliminate one of the trios. Sounds easy, right? It's not.
Prevention is always the best cure, and this means killing the mite, along with its eggs, before it can do any damage. But miticides and other measures commonly used for treating spider mite infestations aren't effective because this is a different type of mite.
It's believed that all cultivated roses are susceptible to the disease; this includes shrubs, hybrid teas, floribundas, grandifloras, and miniatures. So even if there's no evidence of multiflora in the immediate area, other roses may already carry the pathogen and can transmit it to your presumably clean stock. Once that happens, removal and destruction of the affected plants is your only option.
A little background of the disease.
This disease was first detected in the 1930s in wild roses growing in the mountains of California and Wyoming. Then it spread to stands of Rosa multiflora, an almost perfect host, and moved across the country and into the Midwest. In the late 1970s and early 1980s, it swept like a wildfire up the Ohio River valley and into West Virginia. Today, it’s endemic wherever wild multiflora roses are found, including the nearby states of Oklahoma, Arkansas, and Kansas. In Texas, rose rosette was discovered around Tyler in 1990 in fields of multiflora roses being grown for use as rootstocks. It disappeared for a time but was identified in a garden in Mineral Wells in 1997. Then, in 1998, it showed up in Fort Worth and, later that summer, just north of Dallas in a large public rose garden in the city of Plano.
According to Dr. George L. Philley, Extension Plant Pathologist at the Texas A&M facility in Overton, some new type of virus, not yet described, is suspected as the cause, but that’s by no means certain. In any case, he says, it's almost impossible to transmit this disease mechanically, such as with clippers. Rose rosette is carried from plant to plant only by a tiny, wooly mite, Phyllocoptes fructiphilus. "But this isn’t like a spider mite," Dr. Philley says; it’s an eriophyid mite of the same general type that transmits virus diseases in wheat." "It's microscopic in size," he adds, "but can be seen in a 10-power hand lens if you know what to look for — something like a fly larva, translucent to cream-colored, and with 4 legs on one end." On a rose, it takes up residence in the axillary-bud regions under the stipules, where the petioles (stems) of the leaflets emerge from the canes.
What to look for
Symptoms of RRD can be difficult to recognize and confirm, as some signs mimic normal new growth. Many rose varieties are known for their richly colored young foliage, with leaves emerging a deep red to purple in spring. These generally turn green during the growing season, so if a plant continues to display intense red foliage throughout summer, it may have been infected.
Flushes of massed, tangled growth, similar to a witch's broom, can signal the presence of rose rosette virus (RRV). This may be more evident in plants that have been infected for more than a year. Infected plants may display unusually large masses of distorted buds, most of which fail to open. Reddening and/or flattening of the cane can occur. Excessive thorn growth in an obvious sign.
Several of these symptoms appear similar to the sign of herbicide damage - which just adds to the challenge of confirmation.
Plants infected with RRV will decline and eventually die, but the disease may not be fatal for three to four years, by which time nearby plants may have been affected. Some speculate that RRD has become widespread in recent years due to the popularity of mass plantings, positioning plants close enough together to give the mite - which cannot fly, but can "balloon" on the wind and travel great distances - easy access to its next victim. So vigilance is key.
These mites are commonly found in air samples collected for spore and pollen counts. According to Dr. Philley, the numbers of mites in such samples start out very low in the spring, begin to increase by June, peak in September, and then drop to zero after that.
How to manage RRD
Ultimately, the RV is fatal. Once a plant is infected, it will not survive, so management of the disease is about preventing its spread to other plants.
Scouting for symptoms is vital to the long-term survival of nursery stock or landscape installations. It's a good idea to inspect plants before they're purchased and take a look at neighboring plants, as well.
Although selective pruning of affected canes has been suggested as a possible means of management, this has proved - so far- to be ineffective. If signs of the disease are detected, the affected plants must be rogued. Many plants infected with RRV may remain asymptomatic; it's a good idea to remove plants immediately surrounding them, providing a safeguard against further infection. Rogue plants should not simply be removed from the site but bagged where they stand prior to removal.
Miticides have been suggested as a possible "cure," but to date, these methods have been unsuccessful.
The most effective way to fight RRD: Vigilance; early detection; rouging of affected plants.
A quick and dirty checklist of symptoms.
Symptoms can range from what appears to be a flush of new growth to grotesque mutations that disfigure the plant. The most easily spotted signs of a rose rosette attack are:
Increase growth and/or rapid elongation of shoots.
New leaves and twigs have an abnormally bright, rich red color.
Leaves may be distorted, curled, or twisted.
An overabundance of foliage may be produced.
The spiral pattern of cane growth.
Canes produce excessive, red-tinged thorn growth.
Atypical flower coloration, such as mottling.
Deformed buds and flowers.
Witch's brooms are a sure sign of rose rosette infestation, although at first, the odd growth may appear to mimic the effect of herbicide drift. Where canes overproduce thorns, the growth may be so excessive that the stems are covered.