Knockout Roses Just Might Get Knocked Out

By on April 1, 2015

Knockout Roses

Landscape & Garden Editor Kevin Johnson Warns Us About the Deadly Rose Rosette Disease Threatening Our Knockout Roses

Roses, cherished for their beautiful blooms and fragrance, are commonly used in our yards and landscapes. Anyone who has grown roses understands how challenging they can be to maintain, often requiring lots of time and effort to produce, thus enhancing their intrinsic value. The introduction of the Knockout Rose –Rosaceae rosa “Radzazz”– roughly 15 years ago made rose lovers happier than ever as this new variety is known for being easy to grow, extremely disease tolerant, drought resistant and ever blooming.

Now the bad news– Knockout Roses are in trouble. Overplanting and overuse has made them highly susceptible to the spread of insects and disease. Rose Rosette Disease (RRD) may well spell the end of the Knockout Rose as well as several other varieties of roses. RRD or Witches Broom as it is often called because of the way it causes new shoots to grow straight up and close together in broom-shaped clusters, is a viral rose disease spread by a tiny insect. The primary vector of this virus is spread by the Eriophyid mite, which is invisible to the naked eye. The wingless and microscopic mite is carried on the wind, which means mass plantings are particularly susceptible. The virus is spread from plant to plant as the mites feed on the tender new growth of the rose. And while Knock Out roses are its most famous victims, the disease is a threat to all commercial hybrid roses, including favorites such as hybrid tea roses, floribundas, grandifloras and old-fashioned varieties.

RRD can also be spread through lax pruning and grafting practices with the mites moving from plant to plant on tools that have not been cleaned or disinfected between use. Once infected, this disease can be fatal to the plant. I have seen firsthand how devastating the Rose Rosette virus can be, so learning to identify this disease is critical. This spring, it will be extremely important to inspect your roses. Plant symptoms of RRD include a thickening of the stems, the foliage to have an abnormal red color, and rapidly elongating- Witches Broom- shoots. Flowers and buds will also become deformed and the plant may have many distorted and dwarfed leaves.

Rose Rosette Disease Symtoms

  • Thickening of Stems
  • Abnormal red color foliage with Witches Broom shoots
  • Deformed flowers and buds
  • Distorted and dwarfed leaves

If you think your roses have been infected, don’t waste your time and money spraying with insecticides, fungicides and miticides. I’ve tried this approach with little success. Remember we are dealing with a virus that becomes systemic throughout the entire plant. Sadly, RRD is not curable. My advice is to completely remove the entire plant from the landscape, including all of the roots. This may prevent the disease from further spreading to healthy plants. I know this doesn’t sound fun, but it’s the best approach. Once the infected plants are removed, or if your roses are not showing signs of RRD, it is extremely important that pruners be completely disinfected with a water/bleach solution after each use to discourage the spread of the disease.

In the June/July edition of AC Living Magazine I plan on covering the Hemlock Woolly Adelgid as it threatens the survival of this valuable tree species. If you have hemlocks growing on your property, you need to know about this devastating insect. So see you next issue!

kevin_johnson_thumbKevin Johnson is the owner of Green Leaf Lawn and Ornamental LLC, based in Blue Ridge. He was formerly the Grounds Supervisor at Young Harris College and has extensive training in landscape management. You can find Kevin at or reach him toll free at 866-883-2420.