If your roses seem like they are growing a little different than usual, they may be infected with an untreatable disease known as rose rosette.
Infected plants typically produce tight clusters of stems at the end of branches known as witches’ brooms, become excessively thorny, and produce red or mottled yellow foliage. Sometimes leaves are misshapen with an appearance similar to that of a plant with herbicide injury.
Rose rosette disease typically kills infected plants within one to five years of infection. To help prevent spread of the disease on landscape roses, Kansas State University recommends removing infected plants and destroying them as soon as they are noticed.
If you think your roses might be infected with rose rosette, the Douglas County Master Gardeners or your favorite local garden center can help confirm the symptoms from a sample or pictures of your rose plant.
Why does prevalence of the disease seem to be rising? Experts speculate that the influx of shrub roses into the landscape in the last decade is to blame. I agree, although I would also agree that it is unfortunate if you just filled your landscape with those easy-to-grow roses.
On the flip side, rose rosette is considered beneficial in some regards. Multiflora rose, an undesirable species that is classified as noxious in some regions, is highly susceptible to rose rosette, and the plant’s spread has been limited by the disease. And unlike the many insects and disease that have been unintentionally imported, rose rosette is a naturally occurring biological disease in the United States and Canada.
Rose rosette is caused by an organism called a phytoplasma and is most similar to aster yellows disease (also untreatable). Outdated publications may refer to rose rosette as a virus or virus-like organism.
Plant pathologists are still learning about the disease and the causal organism even though it was first discovered in the 1940s.
The disease-causing phytoplasma is most often spread by a microscopic eriophyid mite. Mites travel to new plants on the wind or by hitchhiking on other insects. Females lay an egg a day during their approximate 30-day lifespan.
Mite populations peak in September, although most of the disease transmission occurs earlier in the season when roses are putting on the most growth. Symptoms appear 90 days or more after infection.
Even if your roses look healthy now, keep a close eye on them when they leaf out in the spring. One symptom alone is not enough to verify the disease, but if multiple symptoms are present you may wish to see an expert opinion.
Rose rosette is also spread on infected pruners or when infected plants are grafted to healthy plants. Always disinfect pruners between plants when pruning roses to reduce risk of accidentally spreading the disease. Dipping pruners in a 10 percent bleach solution will work, or you can simply spray the blades with a household disinfectant.
Susceptibility to the disease ranges considerably among rose species and varieties. Multiflora rose is still considered the most susceptible. Some universities report up to six native species of roses with resistance to rose rosette, while other research reports claim some level of susceptibility on all species.
The McCartney rose, which is native to Texas, is resistant to the eriophyid mite that transmits rose rosette but is still susceptible to the disease itself.