Jennifer Schultz Nelson
Extension Educator, Horticulture
Coincidence is a funny thing. Last summer one of our Master Gardeners showed me an article on rose rosette disease, a fatal disease spreading among cultivated roses in the midwestern, southern, and eastern United States. . I agreed wholeheartedly with her that it sure sounded horrible and I was thankful I had never seen it in this area. I think that was what jinxed me.
No more than a week from seeing that article, I noticed a strange looking branch on one of our Knockout® roses. Even if I hadn't read the article on rose rosette disease, this branch would have stood out as strange. A sense of dread filled me as I realized that it looked a whole lot like the symptoms of rose rosette disease I had just read about.
It appeared to be affecting just one branch. That branch started out looking normal, but the leaves and some of the stems were very red. The leaves were elongated, twisted, and red. The flower buds were also twisted and red, twisted to the point that they could not open.
I also noticed a lot of branches emerging from the end of the main branch. This is called a "witches' broom" as it does look somewhat like a broom. This can be a symptom of rose rosette disease, but it can also be a symptom of glyphosate (Roundup®) exposure the previous fall. If the rose was exposed to glyphosate the previous fall, the damage would occur in the buds, which would not be revealed until the following spring as a witches' broom.
I knew that my rose bush had not been exposed to glyphosate, or any other herbicide for that matter. The herbicide 2,4-D can also cause leaf distortion, but again, it was not used near my rose bush. If either of these chemicals were used by a neighbor and had drifted over, you would expect to see symptoms more widespread on the plant, not just a single branch. I was definitely leaning toward this being a case of rose rosette disease.
There is no cure for rose rosette disease. Plants are supposed to be removed and destroyed, preferably burned. I really didn't want to rip out my rose bush. It was one of the first plants we planted at our house. It had grown from a tiny start to a magnificent specimen under my care. Maybe it wasn't rose rosette disease, I told myself.
So while under the veil of Denial, I decided I could prune out the offending branch, and the rose would emerge unscathed. Wrong. A new branch rapidly grew back with all the same symptoms. That was not a good sign.
As if this wasn't horrible enough, it turns out that it is likely that my other roses have rose rosette disease. A plant showing symptoms as drastic as mine are likely to be fairly advanced in the disease.
For years, rose rosette disease was thought to have been a disease of multiflora rose, now considered a noxious weed. Multiflora rose was imported from Japan in 1866 for use as a rootstock in grafted roses. From the 1930's through the 1960's, multiflora rose was purposely planted along highways, as living fences, and for soil erosion control among other uses. Soon people realized this plant was extremely prolific, producing over a million seeds each year, as well as spreading vegetatively through pieces of root or stem.
Multiflora rose is extremely susceptible to rose rosette disease, and the disease is extremely common where multiflora rose is found. It is usually fatal to this rose. Originally it was thought that cultivated roses could not acquire rose rosette disease, but this assumption has been proved wrong. Rose rosette disease jumped to cultivated roses without a problem. Symptoms described as early as 1941 in cultivated roses were probably rose rosette disease.
Currently scientists do not know the exact nature of rose rosette disease, but it is believed to be a virus or virus-like pathogen. They do know that the disease is spread by a tiny eriophyid mite that can travel on the wind. This explains how quickly the disease has spread.
Culturally, rose rosette disease can be controlled by controlling multiflora rose in the vicinity (at least 100 meters) of cultivated roses. Space cultivated roses so they do not touch each other. Remove and burn infected plants. Miticides for the eriophyid mite responsible for spreading the disease may be used, but is only effective and advisable in situations where cultural controls are used.
Thankfully, the rose rosette pathogen is not soil borne, so roses can be planted in the same area once occupied by one with rose rosette disease. However, the disease can persist in any old root pieces left behind from the diseased rose. If you choose to replant your rose, you must be sure to remove any root pieces from the diseased rose.
I am honestly not sure what to do about our situation. I will see if the disease symptoms return this spring, and if they do, finally remove the plant. I am leaning towards not planting another rose in its place, as it seems likely that my other Knockouts® may also have the disease but have not shown symptoms yet.