Rose Rosette Disease

Symptoms

• Although the symptoms of rose rosette disease vary according to the rose variety, common symptoms on most infected roses include elongated stems and clustering of smaller branches or the development of witches' brooms. The leaves of witches' broom are tiny, distorted and feature a red pigmentation. Canes develop excessive growth that is more succulent that normal canes and appears in shades of red. Flowers appear distorted and feature fewer petals than normal. Buds drop prematurely or grow distorted.

Cause

• The rose rosette disease is transmitted by the eriophyid mite (Phyllocoptes fructiphilus), a microscopic, yellow or brown pest native to North America. Unlike other varieties of mites that have eight legs, this vector has only four. The mites are commonly located in the tiny sections between leaf petioles and auxiliary buds. After overwintering on rose canes, adult mites migrate to developing shoots where the females lay eggs. Young mites develop within the leaf folds and remain on the same plant or move from one plant to another. The mites often transmit rose rosette disease between May and July, with symptoms of infections appearing in July and August. Rose rosette disease is also transmitted by grafting or infected pruners.

Chemical Control

• Although no chemical compound exists that directly controls the mite responsible for rose rosette disease, controlling mites with miticides reduces the chances of spread. Because most miticides registered for use on spider mites do not control eriophyid mites, check the label and use one that is specifically registered for control of both mites on roses. Spraying the selected miticide every two weeks between April and September significantly reduces their population, thereby reducing the risk of the disease. You may need to perform an additional spray during the summer, when eriophyid mites are most active.

Cultural Control

• Adopt good cultural practices as soon as you detect symptoms of rose rosette disease. Remove suspect roses immediately to prevent the spread of disease, or isolate and monitor the plant for a few weeks until the symptoms become more established. Do not add infected roses to your compost pile, but bag and discard them, or burn them if allowed in your area. Because multiflora roses are most susceptible to the disease, avoid planting them in gardens, nurseries or greenhouses, or monitor them frequently for symptoms of disease and act immediately to prevent spread.