Pseudonectria buxi causing leaf and stem blight on Buxus in Canada

An outbreak of a boxwood disease was observed in nurseries in southern Ontario in 2008, but the disease appears to have been present in Ontario for at least 15 years. From 2008 to 2010, over 300 fungal isolates were obtained from diseased samples, which included cuttings and whole plants. Almost half of the isolates (144) were found to belong to a single cultural morphotype which was confirmed as Pseudonectria buxi by DNA sequencing of several isolates. The remaining isolates were grouped into another seven major morphotypes, the largest of which accounted for 11 % of the isolates. Single representatives of all eight morphotypes were identified to species using morphological and molecular techniques, and were subjected to pathogenicity testing. Among them, only Pseudonectria buxi successfully satisfied Koch's postulates on detached leaves, small twigs and rooted cuttings, which confirmed it as the causal agent of Volutella blight. Inoculation tests showed that wounds may be the major infection courts for P. buxi, since non-wounded inoculated tissues did not become diseased. Among several commonly grown boxwood cultivars in Ontario, 'Green Gem' was the most susceptible compared to 'Green Velvet', 'Green Mound', 'Green Mountain' or 'Pincushion', but all were found to be susceptible. Rather than behaving just as a saprophyte or secondary invader, P. buxi is capable of primary invasion after wounding, and causing extensive disease on live boxwood tissues.