Symptoms
The symptoms of RRD are variable, depending on the cultivar of rose. Typical symptoms include:
- Elongation and thickening of shoots/stems
- Red leaf mottling (mosaic) that does not disappear as leaves mature
- Leaf distortion
- Excess thorns
- Yellowing and stunting of plants
- Witches’ brooms (rosettes)
- Flower distortion, discoloration, or blight
- Branch/shoot dieback
- Reduced winter hardness
- Increased susceptibility to other diseases
In the early stages, plants may develop elongated stems with reddened foliage, the latter characteristic being typical of new growth with many cultivars. If this foliage does not gradually turn green, the plant is most likely infected. However, this reddening is not a consistent symptom. Elongated shoots may be engorged and pliable with overabundant thorns. Below are examples of what to look for when identifying rose rosette disease.
Look-alike Injury
Many of these symptoms of RRD resemble herbicide drift injury, especially that of Glyphosate, the active ingredient in Round-up®. The commonly-used broadleaf herbicide 2, 4-D can also cause leaf distortion on roses. The only symptoms that may not be present with such poisoning are excessive thorniness and red pigmentation. Such herbicide injury symptoms should disappear in the following year unless the plants are re-injured by drift. Nutrient deficiency may also resemble RRD injury, but will typically affect the whole plant. Therefore, it is important to check for combinations of RRD symptoms over time.
Disease Cycle
RRD, formerly thought to be caused by aster yellows phytoplasma, which also causes witches’-broom-like growths on affected plants, is actually caused by a negative-sense RNA virus called Rose rosette virus, of the genus Emaravirus. The disease is transmitted by the eriophyid mite Phyllocoptes fructiphilus. It can also be transmitted via grafting, but is not sap-transmissible. The virus is not soil-borne, but if infected root pieces remain in the soil after infected plants are removed, they could potentially infect newly-transplanted healthy plants.
Eriophyid mites are microscopic mites that are 3 to 4 times smaller than even an average spider mite. They are so small that they can be transported by wind. They transmit the disease by feeding on infected wild (such as multiflora rose) or cultivated rose plants and being transported via wind and/or clothing and tools of landscape workers to healthy plants, where they feed again and introduce the virus.
Once on a plant, the mite travels very little and tends to hide out in buds, on open flowers and sepals, or at the base of shoots, leaf axils, and in leaf scars during the winter. The virus makes its debut on emerging foliage in the spring, otherwise being inactive during the winter months.