Rose gardeners throughout the country need to be vigilant in watching for the symptoms of an increasingly common problem known as rose rosette disease.
The history and cause
Rose rosette disease (RRD) is fatal, and it has been spreading throughout North America since being first identified in California in the early 1940s. Killing both wild and cultivated roses of most species and cultivars, RRD is considered the most devastating disease of roses. In recent years, hot dry summers have increased the rate of spread and have allowed the disease to move throughout the Midwest.
The causal agent of rose rosette disease is a virus, which may be spread from rose plant to rose plant by any of three methods.
• Roses may become infected if asymptomatic rose buds from stock plants are used in the commercial grafting process, resulting in the production of infected new plants.
• Transmission of RRD is through root grafting, which occurs naturally when roses are growing close together in a bed for several years. The roots will often grow together, or graft to one another, resulting in a shared vascular system through which the RRD virus can pass from plant to plant.
• The most common method of transmission of the virus is by an eriophyid mite (Phyllocoptes fructiphilus). This narrow bodied four-legged mite is three to four times smaller than the more common spider mite, and feeds on new tender growth of rose stems and buds, passing the RRD virus into the vascular system of the plant. Though the mites are flightless, they are blown great distances on upper air currents, to be deposited randomly onto the rose plants of unsuspecting gardeners, resulting in the long-distance movement of the rose rosette virus.
The incidence of rose rosette disease is associated with the spread of its primary host plant, the common and invasive multiflora rose (Rosa multiflora). Multiflora rose was introduced to the United States from Japan in the late 1800s for use as rootstock for cultivated roses. Since that time, multiflora rose has spread uncontrollably and is now considered an invasive species in most states. It is highly susceptible to rose rosette disease.
A large multiflora rose plant may live with the RRD virus for as many as six years, serving as a reservoir for the virus and a host plant for the eriophyid mite until the plant’s eventual death. Initially it was hoped that RRD would wipe out the populations of invasive Rosa multiflora, but this has not occurred. While reducing the population of invasive multiflora rose might be a benefit of RRD, there are certainly no benefits of having RRD in the garden or landscape.
Rose rosette disease in the garden
All species and cultivars of garden roses are thought to be susceptible to rose rosette disease, and as is the case with many viral diseases, there are no cures or treatments for infected plants.
Additionally, the most common vector of RRD, the eriophyid mite, is very tiny and difficult to detect, so gardeners must learn to recognize the earliest symptoms of RRD in order to prevent its spread throughout rose plantings and landscapes.
What to look for
Symptoms of rose rosette disease may vary slightly depending upon which cultivar of rose is infected, but there are several general symptoms that gardeners must watch for.
General symptoms of RRD first appear in spring, intensifying through the summer. These include distorted leaves and growth. This distorted new growth may include brooming, which is dense clustering of small branches into tufts resembling brooms. These brooms and distorted leaves may look like herbicide damage.
RRD will cause distinct red coloration of distorted tissue, as well as the rapid elongation of new red stems. These newly elongated stems will usually be thicker than the parent canes, and will often contain an excessive number of soft and pliable red thorns. Certain rose cultivars, when infected with RRD, will not bloom, and other cultivars may produce small, deformed flowers with mottled coloration.
What to do
Since there are no chemical options for dealing with rose rosette disease, careful cultural management of roses is necessary. If possible, eliminate any wild multiflora roses growing in the vicinity of your plantings. It is critical to recognize that rose rosette disease is systemic, meaning that the virus will be present throughout the entire plant even though symptoms may be seen only on just a few canes or stems. For this reason, pruning out symptomatic growth will not cure the plant or rid it of RRD.
Plants showing symptoms must be removed completely, and it is recommended that the rose plants growing adjacent to an infected plant also be removed and destroyed. Plants to be removed should be bagged or covered before digging them out of the garden. Bagging the plants will help to ensure that any mites present will not be inadvertently transferred to nearby healthy plants.
Keep in mind that root systems will contain the RRD virus, so all root fragments should be removed to prevent resprouting of infected plants. The infected plants can be burned, or should remain bagged for curbside garbage collection. This material should not be composted or added to municipal organic waste collection systems, as the RRD virus may not be destroyed in the composting process.
The mite that vectors the rose rosette disease virus is difficult to detect, making chemical control or prevention very challenging and impractical. While there are a few pesticides that could have some impact on the mite populations, chemical control of mites to prevent the spread of rose rosette disease has not be widely successful and is not as effective as early disease detection and cultural control.
Research is underway to develop better management options for rose rosette disease. Additionally, rose companies and universities are undertaking research in an effort to create roses that are naturally resistant to both the eriophyid mite vector, as well as to the rose rosette virus itself.
Rose rosette disease is a reason for concern, but not a reason to stop growing roses. Purchase new roses from a reliable grower or retailer, closely inspect new rose plants, space plants correctly in the garden, and scout often for RRD symptoms. With due diligence, rose enthusiasts can still enjoy growing their favorite plants.
A version of this article appeared in a May/June 2016 print edition of State-by-State Gardening.
Photography courtesy of Dawn Dailey O’Brien, James W. Armine Jr., and Mary Ann Hansen.