Boxwood Breeding at the United States National Arboretum

Boxwood (Buxus spp.) are slow-growing evergreen shrubs and small trees that are extensively grown as hedges and for topiary. Over 13 million boxwood plants are sold in the U.S. each year, with an annual market value of over $100 million. Boxwood blight disease caused by the ascomycete fungus Calonectria pseudonaviculata was first reported in Connecticut and North Carolina in October 2011, and later identified in many other states and several Canadian provinces. This pathogen causes severe defoliation leading to plant death in nurseries and established landscapes. In response to this problem, a boxwood breeding program was initiated in 2012 at the U.S. National Arboretum in order to develop blight-resistant cultivars. New accessions from domestic and international sources were added to the existing National Boxwood Collection, and SSR markers were developed to assess genetic relationships of this diverse taxa. Parameters for effective in vitro resistance screening were identified to enable rapid screening of hundreds of boxwood accessions and hybrids. Controlled hybridizations were conducted to determine practical breeding parameters such as pollination techniques, self-incompatibility, fruit set, and seed germination protocols. The results of our collection efforts, diversity analysis, resistance screening, and hybridizations will be presented.