Rose rosette disease, also known as witches’ broom of rose, is caused by a virus that is spread by a small eriophyid mite. The disease is limited to plants in the genus Rosa and some rose varieties are believed to have some resistance. Its main host is the multiflora rose, which is considered an aggravating weed by farmers.
The earliest symptoms of rose rosette disease include a red pigmentation of the underside of leaf veins followed by sharply increased growth of vegetative shoots. Leaves often become deformed, crinkled and brittle with yellow mosaics and red pigmentation. As the disease progresses, leaves become small, with bushy lateral buds, producing short intensely red shoots and a proliferation of thorns.
The mite that spreads this virus is a wingless mite that is carried by the wind. They are also spread as they attach to clothing and equipment. The mites are most prevalent in the rose shoots where they feed and reproduce. Females overwinter under bark or on bud scales of living roses. The mites are hampered by low humidity and can only survive about eight hours without being on a host plant.
Virus transmission happens most readily between the months of May through mid-July when plants are making active growth. Symptoms from new infections usually start appearing in mid-July. In general, smaller plants go through the disease stages more quickly than larger plants. Small plants are usually killed in about two years, while a large plant may survive for five years in a deteriorated condition.
It is recommended to remove ornamental roses with symptoms. The entire plant including the roots should be removed and destroyed by burning or placing in a plastic bag. Care must be taken when working with diseased plants as you can facilitate spread of the mites that spreads the disease. Bag the plant before removal, cut it at ground level and then dig out the plant’s roots. Soil need not be removed. Clean tools and put on fresh clothing before moving to a disease-free plant or area.
Plant ornamental roses as far away as possible from known stands of multiflora rose. The general recommendation is maintain at least 300 feet between your roses and any stands of multiflora rose. Even greater distance is preferred especially if they are upwind of your desirable rose plants.
Control the disease by controlling the mite. Start mite control early by pruning your roses hard in late winter (back by 2/3) to remove as many overwintering mites as possible and then spray with horticultural oil to kill any remaining mites. Organic pesticides such as horticultural oils and insecticidal soap are recommended over other pesticides as these organic pesticides are less harmful to natural predators that feed on the problem mites.
Insecticidal soaps and oils have a number of advantages for controlling insects. They are virtually non-toxic to humans and other mammals, and are relatively safe to beneficial insects in the landscape. Apply according to label directions and pay particular attention to the new growing tips where the mites will congregate.
Be aware that both soaps and oils can cause damage to plants if applied when plants are water stressed, temperatures are higher than 90 degrees, or high humidity prevents rapid drying. Some plants are sensitive to oil sprays. Always carefully read and follow the label directions.
Since soaps and oils work on contact, an effective application must coat both the upper and lower leaf surfaces, as well as stems for best results. Repeated applications may be necessary. Apply soap or oil sprays in the early morning or late evening to reduce drying times for more effective insect pest control. Refrain from using leaf blowers around roses as they can spread mites.
Do not plant roses too close together. With extra space between the plants mite movement can be reduced. Also, consider interplanting roses with other ornamental plants. One should be extremely cautious and good neighbor-minded when it comes to rose rosette disease.
For more information, contact the UT-TSU Extension Office in Wilson County at 615-444-9584. You can also find us on Facebook or visit extension.tennessee.edu/wilson.
Through its mission of research, teaching and extension, the University of Tennessee Institute of Agriculture touches lives and provides Real. Life. Solutions. ag.tennessee.edu. Ruth Correll, UT Extension-TSU Cooperative Extension agent in Wilson County, may be reached at 615-444-9584 or firstname.lastname@example.org.