2019 ASHS Annual Conference

Boxwood (Buxus L. spp., Buxaceae) are popular woody landscape shrubs grown for their diverse forms and broad-leaved evergreen foliage. Each year, more than 11 million boxwood plants are sold in the United States, with an annual market value of $126 million. Boxwood plants grown in temperate zones are now threatened by a destructive blight disease caused by the ascomycete fungus Calonectria pseudonaviculata. The disease was first identified in the United Kingdom in 1994 and it has spread throughout continental Europe, parts of western Asia, New Zealand, and into North America. It causes dark brown to black lesions on leaves and severe defoliation leading to plant death in nurseries and established landscapes, hence the need to develop blight-tolerant boxwood cultivars. The National Boxwood Collection at the U.S. National Arboretum contains more than 700 Buxus accessions, making it one of the most complete collections in the world and a valuable genetic resource for developing blight-tolerant varieties. We screened all of these accessions using a lab-based detached leaf assay, and found variability in tolerance, but no complete resistance. We found that in some cases, our results were inconsistent with results of previous resistance screening assays, which were sometimes inconsistent with each other. We performed a meta-analysis using all data sets available producing a list of cultivars sorted by their resistance, along with a measure of resistance variability. In addition, we determined which kinds of assays were most reproducible. Results will enable further development of consistent and accurate resistance screening protocols and indicate the most suitable material for developing more resistant cultivars.