Rosarians know this disease is a problem as soon as they see it. Rose rosette causes plants to form very thick, redder-than-normal stems with four or five times the normal number of thorns. If you miss these symptoms, you are not looking at your roses. When I first heard about the disease, I was told that it would not get into the retail market because it was so obvious that infected plants would be pulled by wholesalers. In fact, this disease is becoming far too common. Infected plants cannot be cured and must be removed from the garden, roots and all. Here’s the facts.
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 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 growing in different directions, giving the plant a witches’-broom appearance (described in hackberry article, this issue). 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 in the garden.
The vector of this disease is an eriophyid 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 would need a strong magnifying glass to see these mites. 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 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 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 Report on Plant Disease RPD no. 666, “Rose Rosette Disease.”