There is one symbol of Halloween that no gardener wants to have in their garden. What is it? Witches broom.
Above: Close up of Witches Broom, A symptom of Rose Rosette Disease
Witches broom, a symptom of rose rosette disease, is the scourge of anyone growing roses. Though it was for many years considered to be primarily a disease of Rosa multiflora, the wild rose which is now an invasive species in many parts of the country, rose rosette disease has become more prevalent in many areas. Earth-Kind® roses, perhaps because they have been planted so extensively, close together, and in large groups, seem to be particularly plagued by rose rosette in the Dallas area. However any rose may be infected.
Above: Rose Rosette Disease affecting American Pillar Rose
Rose rosette disease is often seen in the spring but intensifies as the season progresses. Symptoms are variable but, according to University of Kentucky’s Cooperative Extension, often include increased growth/rapid elongation of the shoots, abnormal red discoloration of the shoots and leaves, witches broom (prolific clustering of small shoots), and over-abundance of thorns. Some of these signs may at first be confused with glyphosate (example: Round-Up) or growth regulator ( example: 2-4-D) herbicide damage. However, rose rosette disease is progressive and plants exhibiting symptoms should be carefully monitored. Unfortunately, sometimes by the time rose rosette disease is confirmed in an affected rose, the virus has spread to other roses near it.
The disease causing agent has only been identified within the past several years. The rose rosette virus, called RRV, is transmitted by an eriophyid mite, Phyllocoptes fructiphilus,, and through grafts. The mite is extremely small and cannot fly. However it can crawl, move from plant to plant by air currents, or attach itself to insects.
Currently there is no effective control available for rose rosette disease in existing infected plants. However the facts that it is caused by a virus and spread by a mite give some clues as to how to prevent its spread to healthy plants. Though miticides have been tried, miticides that control spider mites are not effective against the eriophyid mite. According to the Virginia Cooperative Extension, the insecticide Avid is registered for control of both spider mites and eriophyid mites on roses. However, the use of an insecticide or miticide is not recommended without using cultural control methods as well.
Virginia Cooperative Extension recommends the following cultural controls. Early detection is the key:
1) If any wild roses are present in the area, remove and destroy them. Continue monitoring them for regrowth from roots.
2) Remove any suspected roses by bagging and destroying them. Be sure to remove all the roots, both to prevent re-growth of the infected plant and to keep the virus from spreading to other rose roots nearby. It is not recommended to plant another rose in the same space, though plants other than roses can be grown there with no problem.
3) If you live in an area where wild roses are present, avoid planting downwind as the eriophyid mite can move by air currents.
4) Be sure to consider the mature size of each rose plant and space them so that the canes and leaves do not touch each other. The eriophyid mites do not have wings and must crawl from plant to plant, so proper spacing makes it more difficult for the mite to move from rose to rose.
5) Finally, purchase only healthy roses from reputable dealers. If you detect any signs of rose rosette disease in a plant before purchasing, do not buy it. Remember, “an ounce of prevention is worth a pound of cure.”
If you take all these precautions, hopefully you won’t have a frightful experience with your roses.
Pictures by American Rose Society