The symptoms of RRD are variable, depending on the cultivar of rose (Table 1). In the early stages, plants may develop elongated stems with reddened foliage, the latter characteristic being typical of new growth with many cultivars, but if this foliage does not gradually turn green, the plant is most likely infected (Figure 1). However, this reddening is not a consistent symptom. Elongated shoots may be engorged and pliable, with overabundant thorns (Figures 2 & 3).
Table 1: Typical RRD symptoms
|Elongation and thickening of shoots/stems|
|Red leaf mottling (mosaic) that does not disappear as leaves mature|
|Yellowing and stunting of plants|
|Witches' brooms (rosettes)|
|Flower distortion, discoloration or blight|
|Reduced winter hardiness|
|Increased susceptibility to other diseases|
Another common symptom of RRD is "witches' brooms," brush-like clusters of shoots and branches that originate at the same point. Such growths are also called "rosettes," lending the name to the disease (Figure 4). The foliage within these rosettes may be stunted, distorted and mottled red or yellow. Witches' brooms will spread randomly across the plant as the disease progresses. Flowers may be distorted or fail to open fullly (Figure 5). Severely infected plants may not produce flowers.
RRD will commonly cause defoliation and dieback, and in severe cases plants may die within 2-3 seasons. Infected plants have reduced winter hardiness, making them more susceptible to cold damage. Plants stressed by RRD are also more susceptible to other diseases such as black spot and powdery mildew.
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.
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 winter months.