For those of us with an emotional attachment to our roses, nothing is more devastating than Rose Rosette Disease (RRD). Rosette disease is terrible because there is no cure – when this virus strikes, there is no way to save your plants.
There are a host of other problems that can affect your roses, of course, but RRD – which was first discovered in American gardens in the 1940s – is one of the worst.
This disease, also known as “witches’ broom,” appears on all varieties of roses and can spread quickly through your rose garden. But what is Rose Rosette disease and, more importantly, how can you prevent it?
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What is Rose Rosette disease?
Roses evoke a lot of emotions in some gardeners. In my garden, many roses come from my grandfather’s plants. Every time I see them, I think of her little yard.
There was a pretty lawn of postage stamps with a large pear tree in one corner and yew bushes along the edges. But half the yard was devoted to my grandfather’s roses. In the summer, we sat on the back steps and breathed in the scent of blooming flowers.
That’s why we try to distribute our emotionally important roses by dividing them among family members in different states. If my southern sister loses her roses, I can bring her cuttings from New England.
How is this disease spread?
Rosette disease is caused by rosette virus, a virulent pathogen spread by tiny eriophyid mites. The mites themselves are so small that they cannot be seen with the naked eye. To see them, you would need a powerful magnifying glass.
Eriophyid mites cannot fly, but they are so small that they are often carried by the wind from one garden to another. These mites feed on roses of all species. If eriophyid mites feed on an infected rose bush and then move to a healthy garden, they transmit the virus by feeding on healthy roses.
Since the mites that spread RRD depend on the wind to carry them from rose to rose, cases of rosette disease tend to be clustered. We don’t yet know much about this disease or the mites that spread it, but no cases have been reported in areas with extreme winters.
Northern Idaho and upstate New York are the areas with the harshest winters where this disease can be found.
Northern New England, Alaska, the Dakotas, Oregon and Montana have not reported any cases of RRD. Colorado and Hawaii also have no reported cases. Northern Minnesota, Wisconsin, Michigan’s Upper Peninsula, Northern Nevada, and Northern Arizona are also currently DRR-free.
Perhaps the mites that spread the Rose Rosette virus cannot survive in these harsh climates. We don’t know for sure, but the researchers hope there may be limits to the spread of RRD.
Recognizing Rosette Disease
RRD is called a witch’s broom for a reason. One of the most telltale signs of rosette disease is the vigorous, clustered initial growth of rose canes. These cluster canes grow quickly, but they tend to look ragged and tattered.
New growth is reddish in color – like new growth is in many rose cultivars. But unlike healthy roses, the pink hue never matures to green. Then the new clustered growth begins to produce clusters of “rosettes”. These rosebuds rarely open, if they bloom, the flowers are stunted and deformed.
The stems of roses affected by RRD become thick, red and very prickly. Plants may show one, some, all, or no symptoms.
Rosette disease is deadly, your roses will not survive the virus for long. It causes plants to weaken and fail.
If you are unsure if this is a true RRD infection, contact the USDA for confirmation. Sometimes herbicides can cause damage similar to Rose Rosette disease. If there is any chance that an herbicide has caused the damage, watch for many vigorous shoots. Only Rose Rosette Disease causes such rapid growth of damaged stems.
Since there is no chance of waiting out this disease or curing your plants, it is important to remove infected roses as soon as possible. The longer infected plants stay in your garden, the more likely it is that the disease will spread. It is also important to report your cases of RRD to the USDA.
So much research is still being done on RRD – your case might help. Reports can also help – RRD researchers can give you up-to-date advice on how to reduce the spread of the virus and save your leftover roses.
Dealing with Infected Plants
Rarely, if you catch it in time, the size of infected canes strength limit the spread of RRD. The virus is systemic and usually circulates throughout the plant, even if you don’t see evidence of it.
At the first sign of infection, prune and discard the affected stems. If you see more signs of rosette disease after pruning, discard the entire plant.
Most of us try to eliminate the infection until the whole plant is affected. All this only gives Eriophyid mites the opportunity to infect other roses. If the disease continues to manifest, destroy the plant.
Then avoid planting roses in that area for at least a year. Eriophyid mites cannot overwinter in the ground, but since we don’t know much about this disease, it’s best to play it safe.
Prevention of rosette disease
There may not be a cure for RRD, but there are many ways to reduce the chances of losing your beloved roses to this virus. Pruning is one of the best ways to prevent the spread of RRD.
In the fall, prune your roses and discard the pruned sections. Eriophyid mites overwinter on unpruned roses, so don’t give them that opportunity.
Continuous deadheading of your roses prevents mites from congregating around dead flowers. Trimming and fall pruning are the first steps in preventing RRD. Of course, they can’t guarantee that your roses won’t catch the witch’s broom, but proper pruning and pruning will reduce the likelihood.
What about row covers?
While the idea of rose row covers sounds like a great idea to keep mites away, row covers actually do very little to protect against mites. Eriophyid mites are microscopic. They can pass through row covers. The microscopic size of these pests makes it difficult to use conventional protective measures against them.
But, although row covers rarely work, windbreaks can help. Building or planting barriers between your roses and the wind will reduce the number of blow mites on the roses. One of the best ways to reduce the risk of RRD in your garden is to protect your roses from the wind. Plant non-mite host bushes between rose bushes, use barrier walls or wooden plant guards.
It is also important to avoid creating your own wind around rose bushes. Do not use leaf blowers or fans in your rose garden. Anything that creates a bit of wind helps mites find their way onto your roses.
Space your roses
Since Eriophyid mites cannot fly, proper spacing of plants can help reduce damage from RRD. Rose Rosette disease is only transmitted by Eriophyid mites. You cannot propagate it with secateurs and the virus cannot live in the ground.
So make sure your roses are touching each other. Plant non-host plants between the roses and give each plant a little extra space. Proper plant spacing can prevent the spread of RRD. Even if the disease affects one of your roses, you can prevent it from infecting all of your roses.
wild roses
If you have wild roses nearby and are also concerned about rosette disease in the area, remove the wild roses. Wild roses are beautiful, but they are also an ideal host plant for Eriophyid mites. Wild roses can be infected for a long time and act as a host plant for spider mites while waiting for a parasitic gust of wind to send them into your garden.
If RRD is spreading in your area and so are wild roses, get rid of them both to protect your garden roses.
What about acaricides?
Unfortunately, miticides are not effective against Eriophyid mites. Constant applications of horticultural oil might help slow the spread of RRD a bit, but it is only moderately effective against these mites.
If you have just removed an infected rose from your garden, try horticultural oil. It may save some of the other roses. But don’t expect it to eliminate dust mites completely.
Resistant rose varieties?
At this stage, there are no RRD-resistant rose cultivars. Gardeners hope to change this in the near future, but as of now there are no plants on the market that can resist the Rose Rosette virus.
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