Identifying and Managing Rose Rosette Disease in Cut Roses
Some of you may have noticed odd-looking growth on your field roses in recent years. The symptoms may be due to rose rosette disease (RRD). This disease was first identified in the United States in the 1940s and has become more common over the last three years or so. It is now known to be caused by a virus vectored by a microscopic mite. It can affect all hybrid roses, including the Knockout Series, and many species roses. Read on to find out how to identify, prevent, and manage this disease.
Symptoms and Diagnosis
Symptoms of RRD can vary depending on the variety of rose involved. They include elongated flexible shoots, proliferation of shoots leading to “witches’ brooms”, excessive development of thorns (soft or not), leaf deformation, retention of juvenile red coloration in shoots, flower abnormalities, decreased cold hardiness, and plant death. Not all symptoms may be present in any given plant. In particular, shoot proliferation and leaf deformation can be misleading, since these symptoms can be caused by exposure to low doses of the herbicide glyphosate. If you observe these two symptoms alone, do some sleuthing to see if drift might have occurred. Diagnosis is usually based on symptoms, with confidence levels ranging from definitive (when hyperthorniness is seen) to tentative. There is a molecular test that can be used to confirm the presence of the virus that causes RRD so check with your local extension agent to find out if it’s available in your area.
Cause and Spread
RRD was only recently proven to be caused by a virus, but it has long been known to be transmitted by the microscopic eriophyid mite, Phyllocoptes fructiphilus. These are not the same as the more familiar spider mites. Small size makes up for their lack of wings, and these mites can be carried about on air currents and perhaps by other insects. According to Star Roses and Plants/Conard-Pyle there is no evidence that RRD can be spread by pruning or cutting tools, but it is a good general practice to sanitize knives and shears frequently during pruning operations anyway. Of course propagating from infected plants or grafting onto infected rootstocks would result in infected roses.
Best growing practices always include preventative measures. Examine any new plants to be sure they are symptom-free. That does not guarantee that they are healthy, since symptoms can take from 17 days to 9 months or longer to show up. Be sure to buy plants from a reliable source. Wild multiflora roses are the primary host and are an important source for the virus, so removing any wild populations within 100 to 150 meters of your plants is necessary.
Star Roses and Plants/Conard-Pyle recommends pruning dormant plants just before new growth appears to eliminate or reduce mites and their eggs that hide in bud crevices. They recommend cutting the plants back by two-thirds their size. Given the mobility of the mites and the possibility of root grafts, plant roses far enough apart as possible so that roots or branches don’t touch. This is sometimes hard to justify when field space is at a premium, but this will reduce the chance of RRD spreading to a healthy neighboring plant and potentially ruining an entire planting. If space is limited, try interplanting other species among the roses.
There is no effective chemical treatment for plants infected with RRD or any other viral disease, but controlling the mite vector is a good place to start. Trials at the University of Tennessee are evaluating miticides to control eriophyid mities, and preliminary results look promising. However, the mites are very tiny, making it hard to get acceptable coverage. Pesticide application is certainly no substitute for removing an infected plant from the field. The jury is still out on how to best manage RRD, so the following recommendations are provisional.
Since viruses become systemic in their hosts, pruning may not be sufficient, especially if the disease is not caught early enough. If a bush has only one affected cane, pruning that cane as close to the ground as possible might get ahead of the infection, but we have no definitive data to show that this is effective. Removal of infected plants is the safer course of action. Plants should be bagged before digging or as soon thereafter as possible, to reduce the chance that the mites will scatter on the wind and carry the virus to nearby roses. Remove enough of the roots so that the infected plant does not re-sprout. Fragments of small roots left in the soil after plant removal should pose no risk. As long as there are no other infected roses nearby, replanting can be done immediately. The virus and mites should die quickly after plants are chipped, so properly composted mulch should not be a source of the disease. Finally, keep a close watch on areas where diseased plants were removed, to be sure they do not sprout again.
As plant pathologists and entomologists continue to conduct experiments over the next several years, we’ll be in a better position to know what works and what doesn’t. The ultimate solution will be to have resistant varieties. Perhaps some of the resistance in our native species like Rosa setigera and Rosa carolina can be brought into cultivated types. However, breeding takes time.
Photo 1: A rose with RRD showing the juvenile red coloration, flexible shoots in a witches’ broom, and hyperthorniness.
Photo 2: RRD symptoms showing witches’ broom, flexible shoots, and leaf deformation.
Graduate Research Assistant
Alicain Carlson is a graduate research assistant at North Carolina State University studying cut flower production and postharvest under the direction of John Dole. Contact her at [email protected]
NCSU Plant Disease and Insect Clinic
Michael Munster diagnoses diseases on ornamental crops for commercial nurseries and greenhouses at the NCSU Plant Disease and Insect Clinic. Contact him at [email protected]