Roses

Rose Rosette Disease is the result of a virus spread by a wingless mite, Pthllocoptes fructiphilus— and it only affects roses.

Photo by Mary Ann Hansen, American Nurseryman

Three words, Rose Rosette Disease, strike terror in the hearts of any rose aficionado. The deadliest disease affecting rose, Rose Rosette Disease first appeared in Manitoba in 1940, arriving in the U.S. in 1941 in Wyoming. There it apparently lingered until it arose in Nebraska by the 1960s, and it was clearly a problem for the rose industry. Now, it’s a huge problem for North Carolina gardeners.

Why am I writing about this plague in the middle of winter? Aren’t roses dormant? Last month I had to take down a species rose, Rosa roxburghii, a beloved chestnut rose, which was clearly exhibiting active, distorted growth, proving that vigilance is necessary throughout the year.

Scientists initially thought RRD would fight that weedy scourge, Rosa multiflora, a species rose that has infected our whole state and is terribly susceptible to RRD. Alas, it appears that RRD affects almost all roses — and the industry has no weapons to fight this enemy.

RRD is the result of a virus spread by a wingless mite, Pthllocoptes fructiphilus— and it only affects roses. Unable to fly, these wingless mites travel by wind currents, so it has taken them seven decades to get to the East Coast. They suck on the rose canes, thereby introducing the virus into the plant’s vascular system. All roses downwind from the infected rose are at risk since the mites travel via wind currents.

The signs of RRD are abnormal growth, often red in color. Frequently the new elongated growth may include an excessive number of thorns or a witches’ broom. One caveat: many roses put out new red growth so don’t panic if you see the color red without the distortion. The key to identifying RRD is observing the distortion of growth.

However, in the case of my beloved chestnut rose, it appeared just as elongated growth. When the chestnut rose stops growing in winter the canes are grey in color. This new growth was the vivid green seen in the spring with red shoots beginning to appear. My concern grew because this occurred in December, a month when roses in North Carolina should be slumbering.

Remember that RRD manifests itself in displaying abnormal growth patterns. Once the rose exhibits this deviant growth, you must assume that the whole plant is infected as the virus travels through its vascular system.

The temptation is to cut away the infected parts of the plant, which just gives the microscopic plants more time to get to another rose. Alas, you must cut down the whole plant, bagging it as you go. Do not compost any parts of the plant. Be sure to take out the roots, as the virus cannot survive in the soil alone.

The bagged diseased plant should go out with regular garbage pickup, as it must not end up in the town compost pile. Town authorities are often unaware of the dangers of RRD so be firm in requesting this garbage pickup.

There is no cure for RRD. Currently NCSU does not offer a molecular test to substantiate the presence of the disease. The key to containing it is through vigilance, even when the rose is not in active growth.

Here are some guidelines to follow:

  • Inspect all roses closely before introducing them into the garden.
  • Highly susceptible to RRD, R. multiflora is prevalent throughout the state. Try not to plant roses downwind from this noxious weed.
  • “Disease resistant,” a term used often to describe sustainable roses, does not refer to RRD. The Knock Out rose, while impervious to blackspot, is highly vulnerable to RRD.
  • Spreading the roses throughout the garden is a good idea, as the mites cannot fly. A cottage garden with interspersed roses probably provides greater protection than the traditional rose garden with its two rows of closely planted roses.
  • Infection time can range from three weeks to one year.
  • There are no chemical cures. Miticides do not work.
  • Pruning roses in the spring by one-third of their size may help to eradicate mites and their eggs.

My last caveat is this: inspect your roses 12 months of the year, yes, even in winter. The sooner you can rid your garden of a diseased rose, the better are the chances other roses might survive. Right now, vigilance is the only weapon we have.

You can contact us with questions or comments at info@absentee-gardener.com.

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