Knock Out Rose
tough and disease resistant as these beauties are, they are no
match for rose rosette disease (RRD). The disease has been known
in the United States since 1941, and its history is strongly
linked to multiflora rose (Rosa multiflora). Multiflora rose was
introduced from Japan as a tough, cold hardy rootstock for
fussy, tender hybrid tea and floribunda roses. From the 1930’s
through the 1960’s, multiflora rose was recommended for erosion
control, strip mine reclamation, a living fence for cattle, and
as a highway crash barrier.
We have since
learned that it is a terrible weed, with each plant capable of
producing more than a million viable seeds per plant that are
easily spread as birds devour the fruits and deposit the seeds
in their travels. The plant also reproduces vegetatively,
creating thorny thickets that are difficult to control. It is
considered a noxious weed in many states. Multiflora rose is
extremely susceptible to RRD, and researchers have long
considered using the disease as a biological control for this
invasive weed. Unfortunately, the disease easily jumps from
multiflora to cultivated roses.
awareness of RRD since 1941, the causal organism was only
identified in 2011 by researchers at the University of Arkansas
as a virus, now dubbed the rose rosette virus. In the past,
diagnosis of RRD was based on symptoms, but now that the causal
agent is known, a laboratory test has been developed to confirm
the presence of the virus.
disease is spread by an extremely tiny eriophyid mite,
Phyllocoptes fructiphilus, or by vegetative propagation of
infected roses. The mites are not visible to the naked eye, and
they typically are hidden in the buds. They are easily moved
from plant to plant by the wind, on clothing and tools, and even
on animals and other insects. Given their size and propensity to
hide deep in buds, the mites are very difficult control.
anecdotal evidence that some native species of roses are
resistant to RRD, but more research is needed to confirm that.
Some researchers theorize that any rose with Asian rose ancestry
is susceptible to RRD, but again, research is necessary to
confirm whether or not that is true. Given the incredible
utility and commercial success of Knock Out roses, breeders such
as Star Roses and Plants/Conard Pyle are putting extensive
research into the problem, as are university researchers. Even
if some of our native roses are truly resistant to rose rosette
disease, it will take time to develop hybrids that have the
flower power and resistance to other rose diseases that the
Knock Outs possess.
Symptoms can vary among species of roses and cultivars. These
• While the new growth on many roses is red, it
hardens off to green as the leaves mature. With RRD, they remain
• A proliferation of short shoots near the tops
of the canes (witches brooms). This symptom can also be caused
by herbicide injury, so it is not diagnostic by itself.
• Distorted, stunted leaves.
• Affected stems may be thicker than the stem
they are growing from or grow in a spiral pattern.
• Affected stems may have an unusual number of
• Flower buds may abort, or the flowers are
deformed or mottled.
The concern about
RRD spreads far beyond the United States. Roses account for
roughly two-thirds of the international cut flower industry –
worth over $40 billion annually – and there is great concern
that RRD will spread to other countries where this
labor-intensive industry provides employment to thousands of
For now, we have
no readily available, RRD-resistant roses to recommend that
perform like Knock Out roses. The good news is that RRD does not
persist in the soil once infected roses are completely removed.
The disease will persist in roots that may be left behind, so
thorough removal is critical. Once that is accomplished, it is
fine to replant with clean roses.
While there is no cure for RRD, but there are some tips to
Avoid planting cultivated roses downwind of multiflora rose
infestations. If possible, eliminate multiflora rose from a
300-foot radius of the planting area. Monitor them for regrowth
and remove it as it arises.
Monitor cultivated roses carefully, and be prepared to remove
and destroy any with suspicious symptoms promptly. Burn them, if
permitted where you live; otherwise, double bag them and send
them out with the trash. Never compost them or permit them to
remain near roses you value.
Allow adequate space between roses so that canes do not
intermingle and leaves do not touch each other. Eriophid mites
do not fly, so they must crawl from plant to plant - proper
spacing makes that more difficult for them.
There is a lot of disagreement among experts as to the efficacy
of treating roses for mites. You cannot see them because of
their size and where they feed, so it is hard to know if they
are present or not. Some researchers have achieved decent
control with carbaryl (Sevin), insecticidal soap and
horticultural oil, but those materials must be reapplied
frequently between April and September. Be aware that using
carbaryl may lead to an outbreak of two-spotted spider mites
because that material eliminates beneficial insects that keep
spider mite populations in check. Insecticidal soap and
horticultural oil are much easier on those beneficials.
change their colors
Winter rose preparation