Is This Rose Rosette Disease? - Ask Extension
The knockout roses on my condominium property may have rosette disease but the problem foliage doesn't look exactly like the pictures and descriptions...
Knowledgebase
Is This Rose Rosette Disease? #875027
Asked June 27, 2024, 6:26 PM EDT
The knockout roses on my condominium property may have rosette disease but the problem foliage doesn't look exactly like the pictures and descriptions on your website or other websites. We have about 100 of these shrubs and 30-50 have this problem.
In the attached file 'Good Cutting Disease Cutting', the clipping on the left is the on the same shrub as the one on the right. The one on the left I would consider appropriate growth for a knockout rose. The other has a problem! The attached file 'Closeup Disease Cutting' is zoomed in on a section of the diseased growth.
Can you please confirm what is shown in the picture is rosette disease? If not, what could it be and is there a treatment?
Harford County Maryland
Expert Response
It does look like how Rose Rosette Disease manifests on Knock Out roses. In our experience, infected Knock Outs tend not to get the excessive thorniness for some reason, but the leaf deformities and congested growth still appear. The only other alternative for these symptoms is herbicide damage, though that tends to affect wider swaths of the exposed plant versus isolated branches (at first, before the virus worsens). There is no intervention to mitigate herbicide damage after it occurs, since it can't be flushed out of the plant's tissues. Herbicide damage may wear off in time (affected growth can't heal, but new growth eventually becomes normal again as the residues become inert), but Rose Rosette will only worsen and begin to affect more new growth over time.
While herbicide damage can be pruned off to improve the look of the plant, the pruners should be sanitized between shrubs just in case the symptoms are instead viral. (Avoid bleach, which corrodes the metal blades.) Contaminated sap moved from plant to plant on pruning tools might be one route of transmission, aside from the primary route of virus-carrying mites. Symptoms due to a viral infection cannot be pruned out, in that doing so will not remove the pathogen from the plant, even if it temporarily improves its appearance.
Plants suspected of having RRD should be removed and discarded. Replacement with other roses, none of which have reliable RRD resistance as far as we know, means that re-infection is possible in the future. The mites carrying the virus from some other "wild" plant (probably Multiflora Rose in natural areas, as they are highly invasive and often affected with this disease) could blow into the area again and re-infest new roses.
Non-rose alternatives will not be susceptible to this particular pathogen. There are a wide variety of other flowering shrubs as candidates, given that (we presume, since it suits the roses) the site is sunny, well-drained, and not bothered by deer.
Miri
While herbicide damage can be pruned off to improve the look of the plant, the pruners should be sanitized between shrubs just in case the symptoms are instead viral. (Avoid bleach, which corrodes the metal blades.) Contaminated sap moved from plant to plant on pruning tools might be one route of transmission, aside from the primary route of virus-carrying mites. Symptoms due to a viral infection cannot be pruned out, in that doing so will not remove the pathogen from the plant, even if it temporarily improves its appearance.
Plants suspected of having RRD should be removed and discarded. Replacement with other roses, none of which have reliable RRD resistance as far as we know, means that re-infection is possible in the future. The mites carrying the virus from some other "wild" plant (probably Multiflora Rose in natural areas, as they are highly invasive and often affected with this disease) could blow into the area again and re-infest new roses.
Non-rose alternatives will not be susceptible to this particular pathogen. There are a wide variety of other flowering shrubs as candidates, given that (we presume, since it suits the roses) the site is sunny, well-drained, and not bothered by deer.
Miri
