EVERY NOW AND THEN, a plant comes along that fundamentally changes how we garden. Unfortunately, the same is true for the occasional pest or disease.
Remember the once-ubiquitous red tip? The popular shrubs were greatly overplanted in the 1970s and ’80s. Along came a leaf-spot disease called Entomosporium, and now we don’t grow red tips anymore. We repeated our mistake when the Knock Out rose was introduced in 2000.
So easy to grow, Knock Out roses became the common gardener’s rose. They bloom practically all season and are drought tolerant and disease resistant. Tens of thousands appeared in S.C. landscapes, often in mass plantings—a stunning display when flowering and a dramatic change from how older rose varieties were used.
Unfortunately, a relatively new disease in our part of the country, rose rosette virus (RRV), is wreaking havoc on Knock Outs and other roses. Once a rose is infected, it cannot be cured; the disease is fatal. All roses, not just Knock Outs, are susceptible to RRV. The problem is not the roses but how we use them—or, more accurately, overuse them—in the landscape.
RRV is largely hosted by wild populations of multiflora rose, a non-native, invasive species of rose that grows in thickets and hedgerows. It is spread by the tiny eriophyid mite, which cannot fly but can walk between neighboring plants or be blown up to 100 yards by the wind.
RRV can spread rapidly, sweeping through neighborhoods, even wiping out a mass planting in just two to three years.
One way to prevent infection is to eliminate any wild roses, particularly multiflora roses, near your cultivated roses.
Because there is no cure, it is vital to detect infected roses early, before the virus spreads. RRV-infected plants can be recognized by abnormal shoot growth, similar to herbicide injury. Other common symptoms are listed in the box below the photo.
Any one symptom alone may not indicate RRV, but a combination may. When caught early, the symptoms will often be restricted to just one shoot. If you have any doubt, take a sample and a picture of the plant to your local Clemson Extension office.
Any suspect plant should be removed, roots and all, and destroyed immediately, either through burning (where permitted) or bagging and disposing in household garbage.
RRV is systemic within plants, making it impossible to prune out. It can be spread during pruning, so clean your pruners with a 10-percent bleach solution or alcohol between shrubs.
Soil does not harbor this disease, so you can replant roses in the same area. However, any roots remaining in the soil could harbor the disease. Before replanting, remove all roots or allow enough time for remaining roots to die.
Make sure new rose bushes are disease free and space them far enough apart so leaves and canes do not touch. Spraying approved pesticides to control the mites is possible, but it is difficult to do successfully and not recommended. Since RRV is only known to infect roses, any other type of shrub or flower could be replanted immediately without risk of infection.
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Symptoms of rose rosette virus
When roses flush new growth in the spring, look for:
- shoots that elongate more rapidly and/or are noticeably thicker than others;
- excessively branched stems, known as witches’ brooms;
- distorted leaves that are smaller and more strap-shaped than normal;
- excessive thorniness; and
- excessive red pigmentation of leaves and stems.
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S. CORY TANNER is an area horticulture agent and Master Gardener coordinator for Clemson Extension based in Greenville County. Contact him at shannt@clemson.edu.