The deadly rose rosette disease (RRD) is already making itself known in our wild and ornamental rose population, including Knockouts. Symptoms vary with the type or species of rose, but generally include small shoots, off color new growth with smallish leaves, or a combination of unusual growth or discoloration in any part of the plant, including flowers and thorns. On hybrid teas and some tea roses, the new growth may be lime-colored. The shoots may also have many small thorns such that the stems appear almost “furry”.
Some herbicides can also cause the witches brooms, distorted growth, and discoloration, but they do not cause the prolific production of thorns. In addition, chemical injury would probably appear on all the roses or broad-leafed plants in the area, whereas rose rosette will generally show up first in just one rose bush.
This contagious disease is transmitted by microscopic wingless eriophyiid mites. Eliminating rose rosette through mite control has not been shown to work with any consistency. Insecticides are not appropriate and would be a threat to valued pollinators.
Unfortunately, although RRD does not cause the immediate death of infected plants, it cannot be cured or salvaged. However, the disease may be prevented from spreading by completely removing suspect roses including all the roots. Any root or plant material not removed could play host for future problems. Do not put the plant debris in the compost.
It’s speculated that RRD came into the country with the Multi-flora rose which is very susceptible to the disease, showing some of the symptoms as described. That’s another reason to make a survey and encourage the eradication of invasive Multi-flora from your neighborhood.
Article written by Alison Arnold, Extension Agent, Agriculture, Consumer Horticulture, Master Gardener Volunteers.