Tips to Control Rose Rosette Disease

In order to understand rose rosette disease, you need to know something about its primary host, the multiflora rose. If you spend time outdoors in Pennsylvania, you may be familiar with multiflora rose (Rosa multiflora), an aggressive, invasive plant. Multiflora rose was introduced into the United States from Japan in 1866 for use as rootstock in grafting ornamental roses. In the 1940s, it was further used as a conservation “tool" because of its ability to grow rapidly and almost anywhere, including at strip mines to prevent erosion. Multiflora rose has also been used to create living fences to enclose livestock and provide food and shelter for wildlife. Hundreds to thousands of nutritious fruit (hips) are developed on each plant. Each hip produces an average of seven seeds. The seeds are capable of germination for at least 20 years. In many locations, multiflora rose dominates the landscape as dense, thorny thickets along roadsides, pastures, and in old fields. Its eradication is nearly impossible, requiring a combination of mowing and herbicide applications. As a result of its invasive nature, multiflora rose is listed as a Class B noxious weed by the Commonwealth of Pennsylvania. This classification means that the multiflora rose is widely established but cannot feasibly be eradicated.

Some landowners have been encouraged to find that multiflora rose is highly susceptible to the deadly rose rosette disease. Rose rosette disease is caused by a virus (Emaravirus sp.) carried by a tiny mite (Phyllocoptes fructiphylus). The mite is wingless and moves by crawling or by air currents. They will also hitch a ride on clothing and equipment. The mites feed and reproduce on the tips of new growth. Young mites can reach maturity within a week, depending on temperatures. Multiple generations are produced each year.

The virus carried by the mite causes a red coloration on the underside of leaf veins on new growth, excessive growth of thorns along stems, and a rapid growth of short clusters of reddish shoots called “witches’ brooms." Leaves and even flowers can become deformed, crinkled, and discolored. Branches and shoots will start to die. Infected plants also become extremely susceptible to freeze damage and other diseases such as downy mildew and black spot. Small plants can die within two years while larger plants may survive up to five years. The virus is most easily transmitted during the active period of plant growth from May through mid-July.

While all of this is cause for some celebration among those trying to control multiflora rose, there is concern among ornamental rose growers. Multiflora rose acts as the main host for the rose rosette virus. The virus can be spread downwind to ornamental roses up to 300 feet away. There are no effective treatments for rose rosette disease if found on ornamental roses. Ornamental rose breeders have been working to develop disease-resistant cultivars. However, no ornamental rose is completely resistant yet to rose rosette disease. Gardeners, landscape managers, and homeowners who grow ornamental roses should be vigilant for symptoms of the virus. Here are a few interventions to protect ornamental roses from rose rosette disease:

• Prune roses in late winter or spring. Mites overwinter in flower buds and seed heads (hips). Removing the buds and seed heads can eliminate the mites that are present.
• Do not use leaf blowers around roses as mites can be transported on these air currents and spread inadvertently.
• Site roses in a sheltered location that protects them from prevailing winds.
• Maintain space between rose bushes to reduce the likelihood of mites crawling from one plant to another.
• Eliminate multiflora rose on your property before it becomes established.

If symptoms such as those described above are seen on ornamental roses, the entire plant, including the roots, should be removed and destroyed. Either burn or bag for disposal. Take steps to reduce spreading mites during the disposal process. It is recommended that a bag be placed over the entire plant before removal. Cut the plant at ground level and tie the bag. Then dig out the entire root system and bag and dispose of it, too. Leaving any roots in the soil can keep the virus alive. After the plant is removed, clean your tools and clothing before doing additional gardening tasks.

Visit Penn State News A mixed blessing: Rose disease reducing invasive multiflora rose to learn more about this disease.