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Differential Resistance to Botryosphaeria ribis Among Cercis Taxa

The redbud (Cercis sp.) is a popular ornamental small tree or shrub, valued commercially for its early spring bloom and adaptability to diverse environmental conditions. Despite these characteristics, large-scale production of redbud has been limited, due in part to their susceptibility to a fungal canker caused by Botryosphaeria ribis. We screened 711 plants in 11 Cercis taxa for response to inoculation with B. ribis. The taxa native to North America, C. canadensis and C. occidentalis, were more susceptible than Asian species. A logistic regression of the number of symptomatic plants 10 weeks postinoculation with taxa and size (stem diameter) as independent variables explained 41% of the variation. Sixteen percent was attributable to taxon effects and 36% was attributable to taxon-independent size effects. Size and taxon effects were not completely orthogonal, and taxa with larger mean stem diameters generally had higher percentages of symptomless plants. A high level of unexplained variation (59%) was found, and is likely due to intraspecific variation among seed lots. Comparisons of 11 seed lots of C. canadensis revealed significantly different proportions of diseased plants ranging from 52 to 92% after 10 weeks, but all plants eventually became diseased.