Dread garden disease knocking out Knock Out roses – USA TODAY
A killer disease has set its sights on America’s most beloved landscape shrub, the rose.
Even the seemingly invincible Knock Out roses, with their reputation for superior pest and disease resistance, have succumbed to a virus known as rose rosette disease. And while Knock Out roses are its most famous victims, the disease is a threat to all commercial hybrid roses, including favorites such as hybrid tea roses, floribundas, grandifloras and old-fashioned varieties.
Bill Barnes of Barnes Horticultural Services in Bucks County, Pa., says that the disease has been around for a long time, but has only recently started to appear in cultivated roses.
“It came from an isolated infestation in Wyoming in the 1940s and slowly spread from there,” he says.
The virus, native and endemic to the United States, first appeared on native populations of Rosa woodsii in the western U.S. Since then, it has aggressively attacked the invasive multiflora rose (Rosa multiflora), an agricultural and ornamental pest imported from Japan and planted extensively in the Midwest as a “living fence.”
Its introduction seems to have acted as a land bridge, acting as a host to the virus and allowing it to spread eastward. Within the past six years, RRD jumped to landscape roses.
“The two getting clocked the hardest are the Knock Out and Drift roses – the very latest of the top roses,” he says.
With 40 years experience in the industry, Barnes has recently written about the disease for trade publications and observed that it appears to be specific to roses with Asian heritage.
“Asian roses provided us with huge, showy flowers; an example is that all the yellow roses come from one species of rose that occurs in Iran,” he says, suggesting that inbreeding has contributed to disease susceptibility.
“We’ve ignored the American roses and roses from other parts of the world,” he says, noting that many native roses seem to be resistant.
“There are hundreds of species that haven’t been touched,” he says, citing Rosa blanda, R. palustris, R. carolina, and R. arkansana. “Resistance is highly variable across the board.”
When RRD symptoms started showing up in people’s yards six to seven years ago and customers consulted with their local garden centers as to the cause, Barnes noted that people in the trade were equally mystified by the devastation, which manifests itself several ways.
New growth is often elongated and bright red. Plants exhibit bunchy growth at branch’s end, also red, sometimes called a “witch’s broom.” Leaves, buds and flowers are often distorted and thorns, initially softer than usual, are more abundant. Once infected, plants are highly susceptible to other diseases that affect roses, such as black spot and powdery mildew. All this weakens the plant significantly and it dies in two to five years.
The vector for RRD is an eriophyid mite – wingless and microscopic – carried on the wind, which means mass plantings are particularly susceptible.
“It spreads like wildfire when they’re planted close to each other,” Barnes says. The mites seem to be most active in late summer, but damage won’t manifest itself until many months later.
Nancy Gregory, plant diagnostician for the University of Delaware’s Cooperative Extension, says one of the best times to look for this disease is in early June.
“New growth is often bunched together and red in color, but if healthy, it should expand to normal internode length and turn green. (RRD) will be a very obvious distortion,” she says.
And while RRD might sometimes mimic herbicide damage, its intense red stem color is unique. Unfortunately, according to Gregory, there is no chemical or preemptive treatment for the disease, an unsettling thought for the plant’s future.
Because of this, RRD was the subject of a special summit held last year in Newark, N.J., and sponsored by Conard-Pyle, the parent company of Star Roses, which breeds Knock Outs. A team of about 30 professionals converged to assess the problem, and plan for its management. Mike Dobres, managing director of Nova Flora, the breeding division of Star Roses and Plants, helped organize the conference.
“We’re working with researchers around the country to tackle this in two ways: breeding resistant species and educating those in the industry about best management practices,” he says.
Of the former approach, Dobres says, “It’s obviously going to take a long time.” In the short term, he says, “One of the best tips we can give, backed up by research, is to be vigilant and remove any infected plants.”
Both Gregory and Barnes agree that removal of infected plants is the way to go.
“Rip it out and burn it or trash it in a way it can’t contaminate the rest of the world,” Barnes says.
Gregory concurs, “Experiments with pruning out the affected growth have shown the virus will move to lower portions of the plant, so it doesn’t always work.”
Care should also be taken with garden tools used to prune or remove affected plants; any cutting implements should be disinfected thoroughly in between use.
For rose-loving gardeners wondering how to go forward, Gregory suggests they focus on prevention.
“Purchase healthy stock from reputable nurseries. Once roses are in the landscape, good spacing between plants is important,” she says.
Dobres recommends pruning existing roses back by two-thirds.
“You can still cut them back … and you’ll be fine. Mites can overwinter in the live canes, so by removing the bulk of the foliage in the high areas, you’re liable to remove the mites,” he says.
Should we not plant roses? Barnes is emphatic: “I don’t advocate not planting roses, that’s shortsighted. They add too much to our landscape,” he says.
As for the Knock Out roses now in production at Star Roses and Plants in West Grove, Pa., “they are doing fine,” Dobres says. “We’ve seen it, but we’ve pulled everything out. We’re living proof that (removal) does work.”
To find out more about rose rosette disease, contact your county cooperative extension office or visit www.roserosettedisease.com.
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