(08/15/18) BATON ROUGE, La. — New reports of rose rosette disease were recently confirmed in commercial landscapes in the Shreveport and Bossier City area.
It is a devastating disease of roses and was first confirmed in Louisiana during fall 2015, said LSU AgCenter plant doctor Raj Singh.
All rose cultivars are susceptible, including Knock Out roses and wild multiflora roses.
“Symptoms produced by rose rosette disease are highly variable, depending on the cultivar or species of the rose and the plant’s age,” Singh said.
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,” Singh said.
“Infected canes produce excessive thorns that are green or red and soft in the beginning but later harden off as the disease progresses,” he said. “Presence of excessive thorns, especially on newly infected canes, is one of the most reliable symptoms to help with diagnosing rose rosette disease.”
Infected canes are sometimes thicker than parent canes. Reddening of new foliage and shoots also is associated with rose rosette disease.
“Remember, however, that these symptoms can be used to potentially recognize rose rosette disease, but 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 those caused by other diseases, pests and stressors. Improper use of herbicide, for example, may result in distortion and clustering of new growth that looks like a “witch’s broom.”
Abnormal discoloration and distortion of new foliage have been associated with rose rosette disease, but feeding injury from chili thrips, a significant rose-growing issue in Louisiana, also causes similar symptoms.
Excessive reddening of new growth is a normal characteristic of some rose cultivars.
Rose rosette disease is transmitted by a tiny eriophyid mite or by grafting. The eriophyid mites crawl from plant to plant or move long distances with the wind. Grafting of virus-infected scions on healthy root stock and vice-versa may also result in the virus transmission.
The virus is systemic and can persist in the live roots of infected rose bushes. It is not soil-borne.
There is no cure for the disease.
“Several precautions can be taken, however, to avoid introduction of the disease or to reduce its spread from infected to healthy roses,” Singh said. “Remove infected roses completely, including roots. New growth from infected roots may serve as a source of the virus. Dispose of infected roses immediately by burning. If burning is not feasible, bag the infected roses before removal.”
The wild multiflora rose is highly susceptible to rose rosette disease, and eriophyid mites and may serve as a source for both the virus and the mites. Remove symptomatic multiflora roses that exist in areas close to the cultivated roses, Singh said.
The integrated management of eriophyid mites, including cultural practices, may reduce the population of mites and thus the potential spread of rose rosette disease, he said.
“Start with disease-free, healthy roses and inspect for any rose rosette disease symptoms before purchasing roses,” he said.
Properly space the new roses to avoid mites crawling from one plant to another. Clean tools and other equipment used for pruning.
If you suspect rose rosette disease in your roses, contact Singh at 225-578-4562 or firstname.lastname@example.org.
Knock Out rose infected with rose rosette disease exhibiting “witch’s broom” symptoms. Photo by Raj Singh/LSU AgCenter
Excessive thorns on infected rose canes. Photo by Raj Singh/LSU AgCenter
Thicker daughter cane, top, compared to parent cane, bottom. Photo by Raj Singh/LSU AgCenter