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Title: Botryosphaeria Stem Blight of Southern Blueberries: Cultivar Susceptibility and Effect of Chemical Treatments

item Smith, Barbara

Submitted to: Acta Horticulturae
Publication Type: Peer Reviewed Journal
Publication Acceptance Date: 6/9/2009
Publication Date: 6/28/2009
Citation: Smith, B.J. 2009. Botryosphaeria Stem Blight of Southern Blueberries: Cultivar Susceptibility and Effect of Chemical Treatments. Acta Horticulturae 810:385-394.

Interpretive Summary: Stem blight is a destructive disease of blueberries grown in the southeastern U.S. where severe losses often occur when young plants die in newly planted fields. This disease is associated with wounds, and fungicides have not been effective for its control. The commercial blueberry industry in the southeastern U.S. has been based largely on rabbiteye cultivars, but southern highbush cultivars with their low winter chill requirement, late spring bloom, and early fruit production are being planted along the Gulf coast into Florida. Little is known about the susceptibility of these cultivars to diseases such as stem blight. Most blueberry breeding programs to develop new cultivars adapted to the southeastern U.S. include stem blight resistance as a major objective. Generally breeders rely on natural infection of seedlings in field trials as a means of identifying resistant genotypes, but several protocols have been used to screen large numbers of blueberry seedlings for stem blight resistance. These protocols usually rely on wound inoculation of partially hardened-off current-year stems and disease response is determined by measuring lesion length. The primary objectives of this study were to compare the susceptibility of recently released rabbiteye and southern highbush cultivars to that of the susceptible rabbiteye cultivar, Tifblue, and to evaluate the potential efficacy of fungicides for control of stem blight using a detached stem assay. The susceptibility of 21 blueberry cultivars to stem blight was determined by inoculating detached wounded blueberry stems with mycelium of the pathogen. Three southern highbush cultivars were classified as resistant when compared to the susceptible rabbiteye cultivar, Tifblue, and 8 were classified as susceptible. Cultivars identified as resistant using this protocol should prove to be resistant in the field. These results confirm field observations and previous reports that stem blight resistance is present among commercial blueberry cultivars. Fungicide control of stem blight in the field has been unsuccessful. In two studies reported here the detached stem assay showed that the fungicides, cyprodinil + fludioxonil, pyraclostrobin, azoxystrobin, and fenbuconazole may be effective for stem blight control. Screening procedures using detached stems or potted plants facilitate mass selection of resistant genotypes. Breeders and growers can use the information generated from these screening procedures to choose appropriate cultivars for parent lines and for planting in areas where stem blight is known to be a problem. Fungicides showing potential efficacy in these detached stem assays should to be evaluated in the field.

Technical Abstract: Botryosphaeria 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 21 southern highbush and seven rabbiteye blueberry cultivars was compared using a detached stem assay. Succulent, partially-hardened stems were wounded, inoculated with a mycelial block from a 14-day-old culture 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’ in both years were classified as susceptible or very susceptible and included eight southern highbush cultivars: Legacy, Gulf Coast, Cooper, Jubilee, Biloxi, O’Neal, Magnolia, and Reveille. In a comparison of wound versus non-wound inoculations, lesion length was significantly greater on stems wounded by removing a small section of bark prior to inoculation than on non-wounded stems or on stems inoculated at a fresh leaf scar. The potential efficacy of fungicides for stem blight control was determined using the detached stem assay. In two studies stems treated with cyprodinil + fludioxonil or pyraclostrobin had shorter lesions than stems receiving no fungicide treatment while stems treated with captan, ziram, and tebuconazole had lesions as long as those receiving no fungicide treatment.