Differential Resistance to Botryosphaeria ribis Among Cercis Taxa

The eastern redbud (Cercis canadensis L.) is a popular woody ornamental plant native to eastern North America that is planted in the landscape chiefly for its showy early spring bloom. The genus Cercis (family Fabaceae) contains, depending on author, 7 to 13 species or subspecies that occur in North America, Europe, and Asia (4,7,8). Species range in size from small shrubs to trees, tolerate full sun to shade, and are hardy in USDA Zones 4 to 9 (7,9). Despite the horticultural merits and wide adaptability of the genus, its use in landscapes is limited due to susceptibility to infection by Botryosphaeria ribis. Breeding programs for Cercis have also been constrained due to the difficulty of propagating clonal material. An important first step in breeding resistant plants is to identify sources of resistance to the pathogen. Historically, the principal causal agent of this canker disease was identified as either B. dothidea Ces & De Not. or B. ribis Gross. & Duggar (2). Recent morphological and molecular evidence supports differentiating botryosphaeriaceous taxa with Fusicoccum anamorphs into at least two distinct groups. The redbud canker pathogen that is the subject of our study is now confirmed to be B. ribis, distinct from B. dothidea (2,11,12). Preliminary studies suggest that variation exists between and within Cercis species in susceptibility to canker. We hoped to capitalize on this apparent natural variation in resistance by conducting a comprehensive evaluation of several Cercis species. The objective of this study was to identify taxa (or individuals) exhibiting resistance to B. ribis.