Submitted to: Blueberry Research Extension North American Workers Conference Proceedings
Publication Type: Proceedings
Publication Acceptance Date: June 1, 2004
Publication Date: December 1, 2004
Citation: Smith, B.J. 2004. SUSCEPTIBILITY OF SOUTHERN HIGHBUSH BLUEBERRY CULTIVARS TO BOTRYOSPHAERIA STEM BLIGHT. Blueberry Research Extension North American Workers Conference Proceedings. pg. 193-201. Interpretive Summary: One of the more destructive diseases of blueberries in the southeastern United States is stem blight, caused by the fungus Botryosphaeria dothidea. It causes severe losses in both highbush (Vaccinium corymbosum L.) and rabbiteye blueberries (V. ashei Reade) often killing young plants. Currently there is not an effective fungicide for control of this disease, so growers must rely on cultural practices for management of stem blight. Plant breeders are releasing new blueberry cultivars for the southern U.S. many of which are hybrids between the northern highbush and various native southern blueberry species. The objectives of these studies were to determine in laboratory assays the relative susceptibility of these new cultivars to stem blight and the potential efficacy of new fungicides for its control. Three southern highbush cultivars, Pearl River, Emerald, and Star, were classified as more resistant to stem blight than the rabbiteye cultivar, Tifblue. Stems treated with the fungicides azoxystrobin, fenbuconazole, cyprodinil + fludioxonil, pyraclostrobin or pyraclostrobin + boscalid then inoculated with B. dothedia developed shorter stem blight lesions in the laboratory assay than untreated stems. These results will be useful to growers when choosing which cultivars to plant, extension agents when making recommendations, and to research plant pathologists and breeders making decisions on parent lines for future blueberry crosses and chemicals to field test for efficacy for control of stem blight.
Technical Abstract: Stem blight, caused by the fungus Botryosphaeria dothidea, is a destructive disease of rabbiteye (Vaccinium ashei) and highbush (V. corymbosum) blueberries in the southeastern United States. The susceptibility of 23 southern highbush, seven rabbiteye, and two highbush blueberry cultivars was compared in a two year study using a detached stem assay. Succulent, partially-hardened stems were wounded, inoculated with a mycelial block of B. dothidea, and incubated for 15 days. Disease severity was determined by comparing lesion length with that of the susceptible rabbiteye cultivar Tifblue. Cultivars with a mean lesion length less than that of `Tifblue' were classified as resistant and included three southern highbush cultivars, Pearl River, Emerald, and Star. Cultivars whose lesion length was equal to or greater than that of `Tifblue' were classified as susceptible and included eight southern highbush cultivars, Legacy, Gulf Coast, Cooper, Jubilee, Biloxi, O'Neal, Magnolia, and Reveille. Wound verus non-wound inoculations were compared in 2003. Lesion length was significantly greater on stems artificially wounded prior to inoculation than on non-wounded stems or on stems inoculated at a natural wound, a fresh leaf scar. The potential efficacy of fungicides for stem blight control was determined using the detached stem assay. Stems treated with azoxystrobin, fenbuconazole, cyprodinil + fludioxonil, pyraclostrobin or pyraclostrobin + boscalid had shorter lesions than stems receiving no fungicide treatment.