Evaluating Boxwood Susceptibility to <i>Calonectria pseudonaviculata</i> Using Cuttings from the National Boxwood Collection

Abstract

Accessions in the National Boxwood Collection of the U.S. National Arboretum were inoculated with Calonectria pseudonaviculata in order to determine susceptibility to boxwood blight as part of longer-term evaluations of whole plants. Terminal unrooted cuttings were inoculated with C. pseudonaviculata and symptoms rated. Cuttings showed a wide range in susceptibility. There were significant differences in percent diseased leaves and percent defoliation among the 42 species and cultivars. Cuttings of some Buxus sempervirens cultivars were among those with the highest percent diseased leaves, with eight cultivars showing as much disease as B. sempervirens ‘Suffruticosa’: ‘Scupi’, ‘Pendula’, ‘Rotundifolia’, ‘Denmark’, ‘Handsworthiensis’, ‘Northland’, ‘Arborescens’, and ‘Northern New York’. All others showed significantly less disease, as measured by percent diseased leaves. A number of accessions were contrasted to the less susceptible B. sinica var. insularis ‘Pincushion’ and showed a similarly low level of disease: Buxus ‘Green Ice’, B. sempervirens ‘Decussata’, B. sinica var. insularis ‘Wintergreen’, Buxus, sp. (57950*H), Buxus ‘Green Mound’, B. sinica var. insularis ‘Winter Beauty’, and B. microphylla var. japonica ‘Winter Gem’. A diverse array of germplasm is available in the genus Buxus, and identifying acceptable levels of disease tolerance in cultivars that represent this diversity will contribute to its continued use in ornamental landscapes. Accepted 25 December 2014. Published 26 January 2015.