LABORATORY ASSESSMENT OF BOXWOOD BLIGHT SUSCEPTIBILITY OF BUXUS ACCESSIONS FROM THE UNITED STATES NATIONAL ARBORETUM^©

Boxwood (Buxus) is a very important landscape staple in the Northeastern United States in part because it is an evergreen that is not prone to deer browse. The new disease boxwood blight is caused by Calonectria pseudonaviculata (= C. buxicola), an invasive pathogen first noticed in the mid-1990s in the United Kingdom (Henricot and Culmam, 2002), spreading through Europe and to New Zealand (Crous et al., 2002) thereafter. The disease was first detected in the United States in 2011 in North Carolina and Connecticut (Ivors et al., 2011; Douglas et al., 2012). It has caused serious concern in the nursery/landscape industry not only because it can weaken and disfigure plants, destroying their aesthetic value, but also because infected leaves and stems contain microsclerotia that might persist in soil and organic debris for years (Weeda and Dart, 2012; Dart and Shishkoff, 2015). The ability of microsclerotia to germinate and produce conidial inoculum years after diseased plants are removed from a site makes replanting of boxwood in contaminated field nurseries or gardens difficult. In addition to boxwood, pachysandra (Pachysandra terminalis) has also shown symptoms of C. pseudonaviculata infection in the landscape, presumably originating from inoculum produced on diseased boxwood (LaMondia et al., 2012; Douglas, 2012). Learning which species and cultivars of Buxus are least susceptible to this new disease will be important information for landscape designers, as the disease has shown itself to be highly destructive in gardens where the pathogen has inadvertently been introduced. For this study, we collected cuttings of 42 boxwood accessions from the US National Arboretum in late July, 2013. Some of these cuttings were propagated for planting at different sites in Connecticut, North Carolina, New Jersey, and New York, where they will be either inoculated or exposed to natural infection by C. pseudonaviculata. Two sets of unrooted cuttings were promptly tested in vitro for their susceptibility to C. pseudonaviculata in a dip inoculation, and these results are reported here.