Gardening Q&A: Not even Knock Out flowers can stand up to rose rosette disease


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This week's column is in response to numerous questions I am receiving about rose rosette disease.

As we use more of the colorful, black spot-resistant Knock Out roses in residential and commercial landscapes, this disease is becoming more widespread. It has been confirmed from the Midwest to the Eastern seaboard. The most common request I have gotten is to recommend roses that are resistant to rose rosette disease.

Until the development of disease-resistant shrub and ground cover roses such as Knock Out, Drift, Oso Easy and Flower Carpet, roses were certainly nothing a landscape company would recommend for commercial landscapes. That changed in 2000 with the introduction of Knock Out roses. They bloom all summer and resist black spot -- the perfect low-maintenance, high-impact plants to add color to commercial and residential landscapes alike.

Unfortunately, as tough and disease-resistant as these beauties are, they are no match for rose rosette disease.

The disease has been known in the United States since 1941, and its history is strongly linked to multiflora roses. These were introduced from Japan as a tough, cold-hardy rootstock for tender hybrid tea and floribunda roses. From the 1930s through the '60s, multiflora roses were 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 that are easily spread as birds devour the fruit 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 rose rosette disease, 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.

Despite our awareness of rose rosette disease since 1941, the causal virus was only identified in 2011 by researchers at the University of Arkansas. In the past, diagnosis 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. Symptoms can vary among species of roses and cultivars. These include:

* While the new growth on many roses is red, it hardens off to green as the leaves mature. If infected with the disease, the mature growth remains red.

* 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 thorns.

* Flower buds may abort, or the flowers are deformed or mottled.

Rose rosette 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.

There is anecdotal evidence that some native species of roses are resistant to rose rosette disease, but more research is needed to confirm that. Some researchers theorize that any rose with Asian rose ancestry is susceptible to it, but again, research is necessary to confirm it.

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, it will take time to develop hybrids that have the flower power and resistance to other rose diseases that Knock Outs possess.

The concern about the disease spreads far beyond the United States.

Roses account for roughly two-thirds of the international cut flower industry -- worth more than $40 billion annually -- and there is great concern that the disease will spread to other countries, where this labor-intensive industry provides employment to thousands of people.

For now, we have no readily available resistant roses to recommend that perform like Knock Out roses.

The good news is that the disease does not persist in the soil once infected roses are completely removed. The disease will persist in roots left behind, so thorough removal is critical. Once that is accomplished, it is fine to replant with clean roses.

While there is no cure, there are some tips to prevent infection:

* 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 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 populations in check. Insecticidal soap and horticultural oil are much easier on those beneficials.

Send questions to Sandy Feather by e-mail at slf9@psu.edu or by regular mail c/o Penn State Extension, 400 N. Lexington Ave., Pittsburgh 15208.


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