Witch’s broom symptoms caused by rose rosette disease on Knock Out rose in Bossier City. (Photo by Raj Singh, LSU AgCenter) Excessive thorns that are soft and green on infected new growth of Knock Out rose in Bossier City. (Photo by Raj Singh, LSU AgCenter)
BOSSIER CITY, La. – A devastating disease of roses called rose rosette disease caused by Rose rosette virus has been confirmed for the first time in Louisiana by LSU AgCenter horticulturist Allen Owings and AgCenter “Plant Doctor” Raj Singh.
Rose rosette was found on landscape shrub roses growing in a commercial landscape in Bossier City, Singh said.
“Modern roses, which include our hybrid tea, grandiflora, floribunda and landscape shrub cultivars along with antique old garden roses are equally susceptible to the disease,” Owings said. “Naturalized plantings of the wild species of rose called multiflora are serving as host plants around the country.”
Rose rosette disease produces a variety of symptoms, depending on the variety or species of the rose and the plant’s age.
Several different types of symptoms have been reported on infected roses, but some of the more recognizable symptoms of rose rosette disease include “witch’s broom," excessive thorniness, thickened new canes and abnormal discoloration or excessive reddening of new foliage.
Infected roses produce a cluster of new shoots from a single point on the parent canes. The new shoots elongate rapidly and appear like a witch’s broom.
Infected canes produce excessive thorns that are green or red and soft in the beginning but later harden off as the disease progresses. Presence of excessive thorns, especially on newly infected canes, is one of the most reliable symptoms to help with diagnosing rose rosette disease.
Another symptom that may be used to diagnose the disease is that infected canes are thicker than parent canes. Reddening of new foliage and shoots also is related.
“Remember, however, that these symptoms can be used to potentially recognize rose rosette disease but that positive confirmation of the disease requires molecular testing,” Singh said.
Although rose rosette disease produces unique symptoms on roses, those symptoms can be easily confused with symptoms caused by other diseases, pests, stresses and other factors.
Improper use of herbicides such as glyphosate may result in distortion and clustering of new growth that looks like witch’s broom.
Abnormal discoloration and distortion of new foliage have been associated with rose rosette disease, but feeding injury from chilli thrips, which is a very significant rose-growing issue in Louisiana, also cause similar symptoms.
Similarly, excessive reddening of new growth is a normal characteristic of some rose varieties.
Rose rosette disease is transmitted by a tiny eriophyid mite, or by grafting, Singh said.
The eriophyid mites crawl from plant to plant or move long distances with the wind. The virus is systemic and can persist in the live roots of infected rose bushes, but it is not soil-borne, he said.
Grafting of virus-infected scions on healthy root stock and vice versa may also result in virus transmission.
“Management of rose rosette disease in infected roses is not possible. Once a rose is infected, there is no cure,” Singh said. “Several precautions can be taken, however, to avoid introduction of the disease or to reduce its spread from infected to healthy roses. Check out the LSU AgCenter Publication 3355 for steps to avoid and reduce the spread of rose rosette disease.”
Roses continue to be very popular landscape plants in Louisiana and the South, and this should continue, Owings said.
“It is important to be aware of this disease now affecting our plantings, and rose enthusiasts should attempt to make themselves aware of the symptoms,” Owings said.
If rose rosette disease is suspected, consult Singh at 225-578-4562 or email rsingh@agcenter.lsu.edu.
To see photos of symptoms, click here.
Johnny Morgan