UNIVERSITY PARK, Pa. -- Multiflora rose is one of the best -- or worst -- and most visible, examples of an invasive plant thriving in Pennsylvania's landscape. But for better or worse, a disease increasingly is infecting and killing the thick stands across the state.
Rose rosette disease, first seen in Canada, California and Wyoming in the 1940s, slowly has worked its way across the range of introduced multiflora rose, according to James Finley, Ibberson Professor of Forest Management in Penn State's College of Agricultural Sciences.
Landowners and managers who have battled this invasive plant for years are celebrating while rose growers are lamenting, he noted. "The ubiquitous patches of multiflora rose across the state are displaying symptoms of rose rosette disease," he said. "Depending on your perspective, it's either a good thing or a bad thing."
Multiflora rose was brought to North America in the 1700s from Asia as rootstock for grafting ornamental roses. It was not too long, though, before it was recognized for other values. From the 1940s through the 1960s, many conservation agencies touted the "living fence" for its many benefits.
Planted along pasture margins, it kept cows and horses confined. But more important, it was easy to grow and could be grown almost anywhere -- even on old strip mines.
"It held the soil, and it provided plentiful and nutritious hips," Finley said. "It created dense wildlife habitat. Many wildlife species flourished with its presence. Some departments of transportation even thought the tangles of thorns were useful as crash barriers."
However, it soon became apparent that multiflora rose had the potential to dominate landscapes with its rapidly growing canes, Finley explained. As the name multiflora implies, each rose bush is capable of producing hundreds, if not thousands, of rose hips, and each of these hips contains an average of seven seeds.
The plant's ability to survive is formidable. These highly viable seeds can lay dormant for a long time, up to 20 years, waiting for the right conditions to germinate. Birds and other animals that readily eat the hips can quickly spread the seeds across the landscape.
"Fortunately or unfortunately, rose rosette disease is becoming more common -- it has slowly spread through native, wild and multiflora rose populations, arriving in southwestern Pennsylvania sometime in the 1990s," Finley said.
Researchers at the University of Arkansas isolated the rose rosette virus in 2011, although it had been present for many years. The disease is spread by a type of mite or by grafting, and multiflora rose is very susceptible. Mite populations are lowest in the spring and build through the summer, becoming most abundant in September.Here is the downside to the disease that is attacking the invasive plant: Cultivated roses planted downwind of infected multiflora rose are especially at risk when wind currents move mites. Once infected, roses can show signs of the disease in as few as four weeks. There is no known treatment or cure for infected plants.
Rose rosette disease has many symptoms. It typically is recognized by a rapid elongation of new shoots, which often form clusters of small branches or "witches brooms." The leaves on these brooms are often small, distorted and red in coloration. The canes where brooms occur often will be soft and pliable. Even the thorns have these characteristics, at least for a while.
Flowers forming on these canes also may display deformities. Infected plants often die in one or two years; however, some plants may live as long as four years. Some researchers report that infected canes are more susceptible to damage from low temperatures.
While some landowners will celebrate the loss or reduction of multiflora rose, the mortality of this invasive plant is not a reason to reduce vigilance, Finley warned.
"At least one study has shown that the void left by its demise is rapidly filled by bush honeysuckle and, perhaps, autumn or Russian olive, other invasive plants," he said.
"Some people suggest that we give up the fight against burgeoning invasive plants; others argue that we have to encourage more indigenous plants to support native insects, which feed our native species. If you want to keep invasive plant species at bay on your land, the sooner you act the better. It is much easier to control a few plants."