Susceptibility of Boxwood Accessions from the National Boxwood Collection to Boxwood Blight and Potential for Differences between Calonectria pseudonaviculata and C. henricotiae

Abstract

Forty Buxus accessions from the U.S. National Arboretum National Boxwood Collection were evaluated as potted plants and detached leaves for susceptibility to Calonectria pseudonaviculata (Crous et al.) L. Lombard et al., and nine boxwood cultivars were evaluated against both species of Calonectria causing boxwood blight, C. pseudonaviculata and C. henricotiae. Accessions of B. harlandii Hance, B. sinica (Rehder and E.H.Wilson) M.Cheng, and B. microphylla Siebold and Zucc. had less disease than B. microphylla ×sempervirens, and all had fewer lesions per plant than the 20 B. sempervirens L. accessions evaluated. Variation within species was observed. Of the individual accessions, B. sinica var. aemulans (accession 60705*H), B. sempervirens (36365*J), and B. harlandii (18834*H) were least susceptible, with <10 lesions per plant. B. sempervirens ‘Scupi’ (9548*H), B. microphylla ‘Compacta’ (4899*CH), B. sempervirens ‘Arborescens’ (57953*H), B. sinica var. insularis ‘Pincushion’ (51898*H), and B. microphylla var. japonica ‘Jim Stauffer’ (72213*H) each had <20 lesions. These rankings differ from previous studies that used detached leaf and unrooted cutting assays. Normalizing to account for plant size effects on inoculation and disease increased variability for individual accession rankings but did not result in significant differences in the most and least susceptible accessions or species ranking. Nine boxwood cultivars evaluated against both pathogen species exhibited a range of susceptibility against four pooled isolates each of C. pseudonaviculata and C. henricotiae. Although small differences in disease severity were observed on boxwood inoculated with the two pathogens, there was no interaction of cultivar and pathogen species, suggesting that a cultivar rated resistant to one species was resistant to the other. These results may aid boxwood breeders to develop resistance to boxwood blight.