This disease has been sighted at the University of Kentucky clinic as well as The Morton Arboretum clinic in northern Illinois. We can expect to see it in any part of Illinois now. The University of Illinois Plant Clinic has not seen a case of rose rosette yet this year, but telephone reports of its occurrence have been received.
Rose rosette is caused by a double-stranded RNA, which means that it is a viruslike disease. It cannot be cultured in a lab, and confirmation of the disease by the Plant Clinic will be based purely on symptom-ology. Fortunately, symptoms are very distinct. The new growth appears deep red, both on leaves and stems. Leaves may show crinkling, distortion, or a mosaic of green, yellow, and red. An infected plant produces numerous lateral shoots that grow in different directions, giving the plant a witches’-broom appearance. These shoots are typically deep red and much larger in diameter than the canes from which they grow. Thorns on these stems are more numerous than normal, giving the stem an almost hairy appearance. Plants usually die within about 22 months of infection. Because of the way roses are propagated, rose rosette is often identified in the nursery, and infected plants are rogued before they get into the retail market. Sometimes there are escapes or plants are infected after they are planted in the garden.
The vector of this disease is an eriophyid mite, a mite so small that 20 could fit on a pinhead. Eriophyid mites are much smaller than red spider mites, which are commonly seen on plants. Grafting can also spread rose rosette disease.
Multiflora rose is the most common host of this disease, but it has been reported on cultivated flowering varieties as well. Climbers, hybrid teas, floribundas, miniatures, and a number of old variety roses have been infected. Hybrid teas typically show a color that is more yellow than red. So far, no other host besides rose has been found. Our clinic has seen a few cases of this disease on hybrid roses in the past few years.
Currently, infected plants cannot be salvaged. Plants with symptoms should be dug up and destroyed (including roots) when first noticed. It is strongly suggested that multiflora and garden roses be separated as far as possible from each other. The efficacy of mite control has been questioned in control of this disease, but if miticides are used, research suggests that the critical mite-transmission time is May and June, so concentrate your efforts in those months. For details of this disease, consult RPD No. 666. A Kan-sas State Web site with a picture of rose rosette is listed at http://www.ksu.edu/plantpath/extension/facts/ rose2.html.