Rose rosette virus infection has caused hyper-thorniness on this hybrid tea.
Angelika Swantek
 

Rose rosette is a problem that was misidentified for a long time. We have known that it is a virus disease only since 2011. The only way that Rose rosette virus (RRV) is known to spread is via a tiny eriophyid mite that is largely windborne.   

Rose rosette only affects roses. The unfortunate thing is that it infects virtually all roses in the horticultural trade, including hybrid teas, miniatures and those oh-so-popular shrub roses that are resistant to black spot. This disease was historically used in attempts to manage Rosa multiflora, which, although purposefully introduced, became an invasive weed in the eastern U.S. 

Rose rosette virus is systemic within the plant. Early comments on the disease suggested that individual branches might be pruned off to eradicate an infection. Plant pathologists have now shown that this practice does not have any benefit. Because rose rosette is easily brought into new areas when infected plants are shipped to wholesale growers or garden centers, all areas where roses are grown are likely to be at risk.

Rose rosette affects the stem, foliage and flowers of plants. The stems and leaves of affected rose shoots often are reddened, and this unusual coloration persists into the summer. Leaves can be stunted and strap-like, narrower than usual. Some cultivars exhibit hyper-thorniness. The normal branching form of the plant is often disrupted by the pathogen, so that witches’ brooms are formed: Apical dominance is lost, and the ends of branches take on a cabbage-like, rosetted form. Over time, usually several seasons, roses decline and die. In experiments, symptoms have appeared 17-279 days after exposure to a viruliferous mite. The mites, inconveniently, are invisible without a microscope and considerable patience.  

Monitoring:

  1. Watch for unusually reddened growth on roses. Note that the new growth in the spring temporarily shows red pigmentation.
  2. Check for stunted, narrow, strap-like leaves and enlarged canes.
  3. Check for hyper-thorniness.
  4. Are there witches’ brooms?
  5. Consider whether glyphosate has been used nearby. The use of the herbicide glyphosate can have similar effects on roses to RRV. Accidental exposure to glyphosate in late summer or fall can lead to distorted growth in roses the following spring.  
Rose rosette on a Floribunda rose
Margery Daughtrey

Avoiding:

  1. Inspect all incoming roses for symptoms.
  2. If plants are brought in when dormant, check new growth after expansion in late spring.
  3. Avoid drift onto roses when applying glyphosate herbicide — to avoid delayed symptoms that mimic rose rosette and confuse identification of the problem.
  4. Purchase roses from nurseries with stringent rose rosette management problems.
  5. Remove all multiflora roses from the perimeter of nurseries, greenhouses and garden centers.

Responding:

  1. Get assistance from a diagnostic laboratory when roses appear to have symptoms suspiciously like those of rose rosette: make sure of the presence of the disease before you take steps to eradicate it.
  2. Bag and remove the diseased individuals, in order to remove virus-carrying mites as well as the infected roses.
  3. Assist researchers studying this disease: go to roserosette.org to report the sighting of rose rosette. Photographs and factsheets at this website will also help you make a tentative identification.
  4. Stay tuned for the latest entomological research on promising miticide programs to manage the vector as well as plant breeders' efforts to identify roses that are resistant to rose rosette. Best management practices are constantly being refined for this disease, as it threatens a beloved ornamental.

Editor's note: A shorter version of this article appeared in the June 2018 print issue.

Margery is a plant pathologist specializing in ornamentals at Cornell’s Long Island Horticultural Research & Extension Center. She aims to help growers outwit diseases. mld9@cornell.edu