A foliar disease affecting Jewell, Emerald, and O'Neal cultivars of highbush blueberry (Vaccinium corymbosum L.) was observed in commercial plantings in Tucuman, Argentina during February 2007. Typical symptoms were circular leaf spots that were 10 to 15 mm in diameter, light to dark brown with dark reddish borders, and frequently exhibiting a secondary necrotic area around the original spot. The final appearance was a large necrotic area surrounding a pale, smaller spot. On the abaxial leaf surfaces, white mycelia grew from the border of the initial or enlarged spots toward the healthy tissues. Frequently, black pycnidia bearing an amber rubbery cirrhi were observed. Fewer pycnidia were observed on the adaxial leaf lesions. The fungus was isolated by transferring cirrhi from affected leaves onto petri plates containing potato dextrose agar (PDA). Plates were incubated in darkness at 27°C and light brown concentric colonies with an aerial, effuse, white mycelium developed. After 5 days, dark brown cirrhi emerged from pycnidia in culture. Pycnidiospores were obtuse, hyaline, 1-celled (7 × 2 μm), and grew from the base of pycnidia on short conidiophores. Pycnidia were black, conical, obovoid, averaged 110 × 70 μm wide, and were lightly immersed in the mycelium. Pathogenicity tests were performed by spraying a 106 conidia/ml suspension on leaves of nine blueberry plants (cv. Jewell). Plants were incubated in a moist chamber at 25 to 29°C with a 12-h photoperiod. After 5 weeks, all inoculated leaves had foliar symptoms similar to the lesions previously observed in the field. Reisolation of the fungus completed Koch's postulates. Noninoculated leaves did not develop foliar symptoms. Morphological and cultural characteristics of the fungus conformed to the descriptions of Dothichiza caroliniana (Demaree & M.S. Wilcox) reported to cause “double spot” in blueberry (1,2). Symptoms were consistent with those described for double spot disease (2). To our knowledge, this is the first report of D. caroliniana on blueberry in South America.
References: (1) J. B. Demaree and M. Wilcox. Phytopathology, 37:490, 1947. (2) R. D. Milholland. Double spot. Page 17 in: Compendium of Blueberry and Cranberry Diseases. F. L. Caruso and D. C. Ramsdell, eds. The American Phytopathological Society, St. Paul, MN 1995.
