The Latest on Rose Rosette Disease
Rose Rosette Disease (RRD) is appearing with increasing frequency on local roses. The American Rose Society believes that hot, dry summers encourage the spread of this disfiguring and incurable disease, so its incidence is only likely to increase. Quick identification and removal of affected plants is the only known control method.
The symptoms can be subtle in early stages. Winter, before foliage has emerged, is an ideal time to inspect your roses for signs of the disease. Different cultivars can show symptoms in different ways, complicating diagnosis, but here are some typical symptoms.
“Witch’s broom” growth is a common symptom of Rose Rosette Disease
Affected roses may show unusually rapid elongation of new growth, often with abnormal red pigmentation. This red tissue may be unusually susceptible to frost damage.
Often there is “witch’s broom” growth, comprised of small, tightly spaced masses of distorted branches. (Note that some herbicides can also cause witch’s broom symptoms, so this symptom without other RRD symptoms is not accurately diagnostic unless herbicide contamination can be definitively ruled out.)
Some cultivars develop an excess of unusually soft, dense thorns, resulting in a fuzzy appearance, again often with unusually red coloration.
Affected canes may be unusually thick compared to normal growth. The entire plant may be more susceptible to powdery mildew.
Flowers may also be affected. Coloration may be mottled or washed out, or flowers may be comprised of what looks like leaves instead of petals.
Buds may be deformed, stunted, or die before opening.
Multiple symptoms often are present at the same time, giving the unmistakable appearance that something is “wrong”.
The disease is transmitted only by an eriophyid mite. These mites are so small that they are all but invisible and so light that wind spreads them from plant to plant. Because their presence in small numbers may cause no visible symptoms on roses, until the virus strikes you may have no idea that your plants are in trouble. Eriophyid mites cannot fly, so proper spacing and pruning of roses so that they do not touch will slow their spread (however, as mentioned before, a stiff breeze can carry them from plant to plant).
Pruning does not transmit the disease, but may transmit the mites. When pruning roses always prune healthy roses first, then move to those suspected of being infected.
One of the best preventive strategies is to prune in late winter just before new growth begins. This removes mite eggs and any mites that may be overwintering on the rose. After pruning, treat the rose with dormant oil to smother any lingering mites. In-season pruning to maintain size and reduce overgrowth may also be beneficial, as is regular deadheading of fading flowers as the mites tend to collect around open blooms.
Other prevention strategies include:
Avoid rose-only plantings by interplanting with other plant types.
Do not crowd roses when planting, use proper spacing.
Block wind-borne mites with taller hedges or screening plantings on the windward side of your property.
Maintain rose health with regular pruning and proper fertilization and pest control. Stressed plants are more likely to become infected.
Once a rose is infected, there is no cure for RRD. Pruning of affected growth is likely of no use as the entire plant is infected by the time the first symptoms can be seen. Additionally, any rose downwind of an affected rose is at risk. The only true cure is removal of the affected rose. Do not compost; instead, carefully bag the rose before you begin working on it to limit the spread of mites, cut it off at the soil line, and put in the trash once removed. Then carefully dig up the root ball, bag, and discard also. Rake up any fallen leaves or other debris from the affected rose. As the disease is contained only in the vascular system of the plant itself, there is no contamination of surrounding soil, so soil excavation is not necessary. Healthy roses can be replanted where diseased ones were removed. (If possible, wait a few days to replant. Any lingering mites cannot live long without a host and will die in a few days.) If the source of the infection is not removed, re-infection is likely. If you see symptoms on one rose out of many, it is strongly encouraged to begin a preventive miticide spray program on the remaining roses, and keep a close watch for new symptoms.
Regular pesticide applications of a miticide labeled for eriophyid mites through the growing season may block the disease, but a good knowledge of miticides is required as many readily available miticides do not control eriophyid mites. Horticultural oil can provide some control of mites, but must be applied regularly (as indicated by the label) and coverage must be thorough.
Here's a video showing the unmistakable look of a rose affected with Rosette Disease. The witch's broom growth, excess thorn growth, and very red tinted growth are all clearly seen. The flower buds are also washed out to white. You can see that overall, it just looks wrong. Note that Katie refers to the disease as "Rosetta". This is the same disease, it is occasionally referred to as "Rose Rosetta".