Rose rosette, also called witches’ broom of rose, causes the plants to form very thick, redder than normal stems with many times the normal number of thorns. Symptoms are obvious. You might think that your plants have been affected by a herbicide, but other nearby plants are not affected. The disease seems to show in spurts, possibly related to increases in population of the eriophyid mite vector. Infected plants cannot be cured and must be removed from the garden, roots and all.
Rose rosette is caused by a double-stranded RNA, which means it is a viruslike disease. It cannot be cultured in a lab; but fortunately, symptoms are distinct. The new growth usually appears deep red, both on leaves and stems. On some cultivars, the infected growth is an odd green color, as with a nutrient stress. Stems are stubby, soft, and brittle, with deformed leaves that 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 pulled before they get into the retail market. Sometimes, there are escapes, or plants are infected after they are 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; but you can see them with a 10x or stronger power magnifying glass. 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. 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. Concentrate your efforts in those months.
A few have asked whether rose rosette could be used as a biological control for muliflora rose. Although this might work in theory, the chance of causing infection to nearby desirable roses is great and should prohibit this practice. For details of this disease, consult Report on Plant Disease (RPD), no. 666, “Rose Rosette Disease.”