Reoccurrence of Rose Bloom Caused by Exobasidium oxycocci in Cranberry in Massachusetts.

Cranberry (Vaccinium macrocarpon Ait.) vines (cv. Howes) from a commercial cranberry bed on Nantucket Island displayed typical symptoms of rose bloom disease in June, 1997. The affected area (1.5 × 30.0 m) consisted of less than 1% symptomatic uprights and was not covered by sprinkler heads of the chemigation system. The same area that did not receive insecticides or fungicides was damaged by black-headed fireworm (Rhopobota naevana) feeding during the 1996 growing season. The surface of the leaves on the abnormal branches displayed the typical white, powdery external appearance, which consisted of basidia and basidiospores of the pathogen Exobasidium oxycocci Rostr. ex Shear that were hyaline, fusiform with a slight curvature, and measured 14 to 18 × 2 to 3 μm, matching a previous description (2). Plants showing symptoms had been infected during 1996. Abnormal lateral shoots with swollen pink leaves grew from infected axillary buds on the previous year's wood (3). Cranberry plants were inoculated with basidiospores but symptoms may take longer than 1 year to develop. Rose bloom was formerly a common disease affecting cranberry in Massachusetts and control strategies were addressed in the disease management recommendations sent out by the Cranberry Experiment Station to growers through 1954. Bergman (1) reported that the disease was found almost every year before 1945, but since then there were no serious outbreaks, possibly due to changes in cultural practices. The fungicide ferbam (ferric dithiocarbamate) largely replaced Bordeaux mixture as a fungicide to combat fruit rot in the late 1940s. The disease was not brought in with imported vines from the Pacific Northwest, where the disease is common, because the vines in this bed were planted in 1910. The disease has not been observed in either wild or abandoned cranberry beds that have not received fungicide applications. This is the first report of this disease in Massachusetts in nearly 50 years. References: (1) H. F. Bergman. 1953. Yearbook of Agriculture, p. 792. (2) F. L. Caruso and D. C. Ramsdell, eds. 1995. Compendium of Blueberry and Cranberry Diseases. American Phytopathological Society, St. Paul, MN. (3) C. L. Shear et al. 1931. U.S. Dep. Agric. Tech. Bull. 258.