Rose rosette has appeared in Illinois this year. It is a viruslike disease that causes the plant to form very thick, redder than normal stems with many times the normal number of thorns. Multiple stems at the ends of branches produce a witches’-broom growth. Symptoms are very obvious. You might think that your plants have been affected by a herbicide, but other nearby nonrose species 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 that it is a viruslike disease. It cannot be cultured in a lab; but, fortunately, symptoms are very distinct. The new growth usually appears deep red, on both 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 22 months of infection.

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. You can see these with a 10X- or stronger-power magnifying glass. Grafting can also spread rose rosette disease. This Texas Web site, http://froebuck.home.texas.net/newpage2.htm, shows some symptoms of rose rosette, as well as an image of the mite vector.

Multiflore, climbers, hybrid teas, floribundas, miniatures, and some old variety roses have been infected with rose rosette. 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 recent 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, consult Report on Plant Disease, no. 666, “Rose Rosette Disease.”