Rose Rosette or Witch's Broom by Sonja Lallemand

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As spring marches on, rose lovers and rose enthusiasts alike begin the annual ritual of uncovering the rose garden to assess winter damage, clean the beds, and start the tedious task of pruning as the plants come out of dormancy."The keys to effective cultural control when growing great roses with very few diseases are early detection of diseases and the use of resistant cultivars," said University of Illinois Extension horticulture educator Sonja Lallemand.

For rose growers, producing a perfect, unblemished bloom with great foliage is a source of pride. "However, one disease that can kill this dream is the devastating Rose Rosette Disease (RRD)," Lallemand said.This disease commonly known as "Witches' broom" was first mentioned in 1941 in the United States. RRD is most often noted when a clustering of small, distorted, reddish branches appears on affected plants. Lallemand explained that the spread of the disease has been linked to the multiflora rose—a plant from Japan that was introduced in 1866 and is now an invasive pest on the continent. "The multiflora rose is highly susceptible to RRD but also serves as a source of inoculum. In areas where rose gardens are downwind from wild roses and other infected plants, the disease has been decimating rose plantings within a two-year period. That is why early detection is important," Lallemand said.

Recent research has identified the Rose rosette virus which has been spreading through rose gardens, as well as in the wild rose population in the Midwest, southern, and eastern U.S. The eriophyid mite, Phyllocoptes fructiphilus, has long been associated with the spread of the disease while the identification of the pathogen is fairly recent. The mites do not attach themselves to the plants but can crawl from plant to plant (close proximity) and travel on wind currents easily vectoring the virus from infected to healthy plants."At present, there is no effective control for RRD in infected plants," Lallemand said. "Because Rose rosette virus and the mite both require living plants to prosper, the disease may be prevented from spreading by using a combination of cultural practices.

"Lallemand highlighted these considerations:

- In early spring, the symptomatic canes are easily noticed. Symptoms that plants are infected include: 1) new shoots elongate quickly followed by a clustering of distorted reddish branches; 2) the red pigment persists, whereas in healthy plants it disappears as the leaves mature; 3) in some cultivars, the affected canes develop excessive growth of red or green thorns that may stiffen later; 4) affected canes may also be noticeably thicker than non-infected canes and may also grow in a spiral pattern; 5) buds may abort and flowers are distorted (fewer petals) as well as mottled instead of solid color.

- Diagnosis of RRD is very straightforward when all these symptoms are present. However, not all plants will exhibit all these characteristics. Any plant suspected of having RRD must be removed and disposed of. "The best approach is to bag the plant in the garden to avoid dispersing the mites, cut the plant at the base, and dig up as much of the root system as possible," she said. "Keep the rogue plant in its enclosed bag for a few weeks so the mites will die off. In areas where burning is permitted, this method can be used to destroy the plants. If burning is not allowed, the plants should be kept bagged and removed."Early detection, pruning, and removal of single affected branches have been successful in some instances and have saved the plant. To prevent further infections of new plants, removal of diseased roots is important.

Cultural Control - Replanting in previously affected areas can be successful only if proper cultural measures such as proper plant spacing, judicious monitoring and pruning, proper fertilization and watering to produce healthy plants, and the use of resistant cultivars are implemented."If you are in doubt of an RRD infection, contact your local U of I Extension office and submit a sample to the Plant Clinic for proper diagnosis," Lallemand said.

Photo by Missouri Botanical Garden