Tuesday, July 21, 2009

Landscape and Nursery - Rose Rosette Disease

We are seeing more cases of rose rosette disease in Delaware. The following is more information about this disease.

Rose rosette is endemic and eventually lethal to multiflora roses, and for that reason it has been considered for use as a biocontrol. All cultivated roses are considered to be potentially susceptible, though tolerance is variable, depending on species and cultivar.

Rose rosette is caused by a virus or a virus-like organism, according to the most recent information available. It is spread by the eriophyid mite, Phyllocoptes fructiphilus, also known as the wooly mite. Eriophyid mites are unable to fly, but they are so small they are easily blown by wind. Therefore, to help control their spread, ornamental roses should not be planted downwind of existing areas of multiflora roses. The disease can also be spread by grafting. Transmission is most likely in late spring and early summer.

Symptoms of rose rosette are variable, and may begin as a red mosaic pattern on new leaves, followed by a growth spurt resulting in red pigmented vegetative shoots producing stunted leaves and short, densely-packed red shoots in a witches’ broom. Especially on ornamental roses, many more thorns develop, and the new shoots are thicker and more succulent than normal, leaving them prone to frost damage. These plants are also more susceptible to powdery mildew. The original shoot infections will spread to roots, and then to the remaining canes.

Symptoms continue with distorted flowers with fewer petals and abnormal flower color and possible mottling. Buds may abort, deform, or develop into leaf tissue. All this may resemble effects of herbicide injury, especially glyphosphate and 2,4 D, but the plant can outgrow the herbicide injury.

Management: Remove and destroy roses with symptoms, including the rootstock. Space cultivated roses so they do not contact each other. It is considered safe to replant roses in the same area as long as all roots have been removed. Chemical control consists of miticides effective against eriophyid mites. Recommendations are to apply these weekly in June and July. Miticides for spider mites may not be effective against eriophyid mites, so check the labels carefully. Control materials include Avid, bifenthrin, horticultural oils, and insecticidal soap. Use only pesticides registered for your crop and follow all label instructions.

Reprinted from an article by Penny Wilkow in the July 17, 2009 edition of the TPM/IPM Weekly Report for Arborists, Landscape Managers & Nursery Managers from the University of Maryland Cooperative Extension.

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