Rose rosette is believed to be 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 is based purely on symptomology. 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 22 months of infection.
The vector of this disease is an eriophyid mite, so small that 20 could fit on a pinhead. Eriophyid mites are much smaller than the red spider mites that are commonly seen on plants. The disease can also be spread by grafting.
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 fashioned” 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. We have seen a few cases of this disease on hybrid roses.
Currently, infected plants cannot be salvaged. Plants with symptoms should be dug up and destroyed (including their roots) when the disease is first noticed. We strongly suggest keeping multiflora and garden roses as far as possible from each other. The efficacy of mite control has been questioned in control of this disease. If miticides are used, research suggests that the critical mite transmission time is May and June, so concentrate your efforts in those months. Sprays now are of little benefit. For details of this disease, consult Report on Plant Disease No. 666.