Rose rosette disease has become a serious concern for many rose gardeners. Unlike other fungal or bacterial diseases, it is not treatable or curable. The disease only affects roses (Rosa spp.). Depending on class, variety or species, roses are hardy throughout U.S. Department of Agriculture zones 2 to 11. In most cases, an infected rose can be easily identified. Rose rosette disease causes abnormal growth and discoloration, often appearing as dense clusters of rapidly growing foliage, called a "witch's broom." Some roses may also show dark red or purple foliage with misshapen leaves and thick, excessively thorny new canes. Rose rosette disease is most common in central, southern and eastern areas of the U.S., but has been recently documented in Canada and California.
Rose Rosette Disease and Its Spread
Rose rosette disease is caused by a virus or virus-like organism, and is spread by three specific means. Phyllocoptes fructiphilus is a species of microscopic mites that commonly affect roses and is a known disease vector. These mites travel on winds that carry them from one plant to another. They transmit rose rosette disease infection when they feed on plant tissues. Grafting, budding and propagation of infected nursery stock can also spread infection. Lastly, rose rosette disease can spread via root systems from one rose to another, if grown in close proximity.
The Origins of Rose Rosette Disease
Multiflora rose (Rosa multiflora) is a wild rose species, native to Asia, that has naturalized and become an invasive exotic in many parts of the U.S. It is winter hardy throughout U.S. Department of Agriculture zones 5 to 8. During the late 19th and early 20th centuries, this species was commonly used as rootstock for rose-grafting. It was also planted widely and naively by the U.S. Department of Agriculture as "living fences" in agricultural areas. Once multiflora rose was recognized as invasive, plants in the wild were purposely infected with rose rosette disease as a bio-control measure. Wild stands of multiflora rose now serve as host reservoirs for the disease, which can spread to ornamental garden roses via wind-borne mites.
Symptoms and Appearance
Roses affected with rose rosette disease do not show any symptoms of infection at first. After several weeks to months, abnormal foliage and growth patterns become evident. These can be localized to a single cane or stem, but may affect several canes or an entire plant. Excessive growth of dark purple or deep red foliage, likened to the shape of a broom, is common. Other symptoms include extremely thorny canes, often thicker than normal canes, deformed blooms and a declining plant vigor. Rose rosette disease is fatal; if an entire rose shrub becomes infected, it will eventually die.
Rose rosette disease usually travels slowly down the tissues of infected canes. If you spot the disease early enough, prune an affected cane a few inches below the areas of abnormal growth. This may prevent spread of the disease to the rest of the plant. Inspect roses regularly for any abnormal growth patterns. Badly infected plants should be removed from the landscape, including all roots, and destroyed. Any nearby stands of multiflora roses should also be destroyed. Do not replant roses where infected specimens have been removed. Disinfect pruning tools between use to help prevent inadvertent spread of the disease organism to other roses.