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Rose Rosette Disease

Rose virus causes strange growth

By: Sandy Feather ©2013
Penn State Extension


Q. I have a steep bank in my front yard that was dangerous to mow. So I got rid of the grass and planted the bank with Knock Out® roses. Maintenance has been much easier, and it looks stunning. I chose the Knocks Outs® because of their disease resistance and because I think they are lovely. But last year, I kept noticing strange growth on some of the plants. Some of the stems had an unusually large number of thorns, and bright red growth on those stems seemed to be stunted. Can you tell me what it is, and if it is something I should be concerned about?

A. While Knock Out® roses are resistant to common rose diseases such as black spot, they are as susceptible as any other rose to rose rosette disease. This disease has been spreading through wild multiflora rose populations in the mid-western and southern United States since the 1940’s, and now has spread throughout the east. Multiflora rose (Rosa multiflora) is native to China, Japan and Korea. It was introduced to this country in the late 1800’s as an ornamental and for use as a hardy rootstock for grafting more tender varieties of roses.
  


History of Multiflora Rose

In the 1930’s and 40’s, multiflora rose was promoted by the United States Department of Agriculture Soil Conservation Service for erosion control, strip mine reclamation, and use as a living fence because of its dense, thorny growth habit. We now know that it is a terrible weed, due to its ability to grow and thrive in less than ideal situations and its ability to produce prolific seed crops.

Multiflora rose is extremely susceptible to rose rosette disease, and it kills infected plants in about two years. As a matter of fact, the disease is so efficient at killing multiflora rose that it has been considered for use in controlling this pest. Unfortunately, rose rosette disease can also infect our garden roses, including hybrid teas, floribundas, miniatures, climbers, shrub roses and old-fashioned garden roses.

rose rosette disease

Despite the fact that rose rosette disease had been known here for so long, the true identity of the causal organism was not known for certain. It had long been thought that the culprit was a virus, but other organisms such as phytoplasmas had also been considered. That changed in 2011 when researchers at the University of Arkansas and Oregon State University published their findings, confirming that the disease is caused by a virus known as rose rosette virus (RRV).

 

An extremely tiny (1/100 of an inch long and 1/400 of inch wide) eriophyid mite, Phyllocoptes fructiphilus, spreads the virus from plant to plant. They typically feed on shoot tips and at the bases of leaf stems (petioles). These mites are wingless, and are blown from plant to plant by the wind. The virus is also spread when infected (but asymptomatic) roses are grafted to propagate named varieties of roses.

Rose rosette disease is systemic and has been found in all parts of infected plants except for the seeds. The causal agent is not soil-borne, so roses can be successfully replanted where infested roses have been removed. However, removal must be very thorough so that no pieces of infected root are left behind that could re-sprout.

There is still no definitive laboratory test for the disease, so diagnosis is based on a range of symptoms that vary from species to species, even cultivar to cultivar, and with different stages of infection.

Symptoms on multiflora roses include:

• New growth at inappropriate times of the year.

• Rapid stem elongation.

• Bright red to dark red mosaic patterns on the leaves. While many varieties of roses have red-colored new growth, it hardens off to green. With rose rosette disease, it stays red.

• Infected plants produce numerous lateral shoots, known as witches brooms, that create bunches of growth at the tips of canes. These shoots are often bright red.

• Leaves are smaller than normal, and may just feather out beyond the stem, or they may be badly distorted. The distortion is similar to symptoms of herbicide damage.

• Some canes produce only sparse red and/or yellow foliage; others die and turn brown.

Symptoms on garden roses include: 

• Rapid stem elongation.

• Some cultivars and species show red to pink colored leaves; others do not.

• Certain canes produce an overabundance of thorns (hyperprickliness). These canes usually die back in the fall.

• Short, deformed canes grow, often red in color, and produce more buds than usual and smaller-than-normal or badly distorted leaves. Again, the distortion is similar to herbicide damage.

• Flower parts on infested canes are distorted, sometimes leaf-like, while other canes on the same plant produce normal flowers and fruit.

• Powdery mildew becomes a problem on roses that are normally resistant.

• Symptoms come and go. At times, the plant may seem to recover. Many garden roses can go on like this for four or five years.

Protecting your Roses

Although there is no chemical control for rose rosette disease, there are steps you can take to protect your roses. First of all, monitor nearby populations of multiflora roses - especially those upwind of your garden - for the symptoms described above. Since they are so susceptible, they can act as an early warning of the disease’s presence in your neighborhood. If possible, eliminate any multiflora roses growing within 300 feet of your garden. Since multiflora roses do not die quickly from the disease, they serve as a reservoir of infection.

The best course of action is to remove and destroy garden roses that show any of the symptoms described above as soon as you notice them. Be sure to send them out with the trash or burn them rather than composting them.


Mites on Roses

Controlling the mites is not as easy as it sounds – they are active from May through September, they are not visible to the naked eye, and they reproduce very rapidly, especially during hot, dry weather.

While there are recommendations for commercial nurseries to control the mites, the most effective materials are not available to home gardeners. Since they are not insects, traditional insecticides that home gardeners use may not control them. What’s more, those insecticides can kill beneficial insects that keep other rose pests in check, so you can wind up with an outbreak of another pest.

Monitoring your roses carefully and getting rid of any that show suspicious growth is the most practical course of action.


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