Knockout Roses have been all the rage for the last few years. Even as a non-rose person, I have been impressed. No black spot (or very little) and pruning? Prune whenever you feel like it! The plants will live and bloom and bloom and live!
However, like all good things, they must come to an end. Case in point: Several calls and many rose specimens in the disease diagnostic lab with weird, red, thorny growth.
All the Knockouts here at our office were removed due to this problem, a disease problem. You might say ‘How? Knockouts are disease resistant!’.
Nope, sorry, they are not resistant to rose rosette.
Symptoms. This lethal disease affects all weedy and cultivated roses. Earliest symptoms include increased growth of shoots with red coloration and distortion and dwarfing of leaves (Figure 1). Affected shoots appear to be more succulent than normal and they develop a proliferation of thorns (Figure 2). This abnormal overabundance of thorns is a useful field symptom for diagnosis, because the new shoots of many roses are naturally reddish colored. Diseased shoots are not winter-hardy and will produce few blooms or flowers may be deformed. Infected plants produce fewer roots than normal. The disease progresses to the rest of the plant until all the new growth is affected and the plant declines or is killed in winter. Roses may succumb in just one season, or symptoms may continue for another season or two.
The cause of rose rosette disease was thought to be a virus, but recent research suggests that a phytoplasma infection is responsible for the symptoms. In nature, it is spread by a tiny eriophyid mite.
Disease management. Infected plants must be removed and destroyed so that the pathogen is not spread to healthy plants nearby. Care must be taken to avoid scattering disease-carrying mites to the other plants. Early detection is essential. Rose rosette disease is normally systemic in the plant, but at the first indication of infection on a shoot, it might help to clip off the affected shoot in hopes that the rest of the plant is still unaffected. Multiflora rose could be a reservoir for the disease so they should be removed from the neighborhood of cultivated roses.
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