Population Genomics Trace Clonal Diversification and Intercontinental Migration of an Emerging Fungal Pathogen of Boxwood.

Boxwood blight was first documented in Europe, prior to its recent colonization of North America, where it continues to have significant negative impacts on the ornamental industry. Due to near genetic uniformity in the two sister species of fungal plant pathogens that cause boxwood blight, understanding historical disease emergence and predicting future outbreaks is limited. The goal of this research was to apply population genomics to understand the role of pathogen diversification and migration in disease emergence. Specifically, we tested whether the primary pathogen species Calonectria pseudonaviculata has remained genetically isolated from its European-limited sister species C. henricotiae, while diversifying into clonal lineages that have migrated among continents. Whole-genome sequencing identified 1,608 single-nucleotide polymorphisms (SNPs) in 67 C. pseudonaviculata isolates from four continents and 1,017 SNPs in 13 C. henricotiae isolates from Europe. Interspecific genetic differentiation and an absence of shared polymorphisms indicated lack of gene flow between the sister species. Tests for intraspecific genetic structure in C. pseudonaviculata identified four genetic clusters, three of which corresponded to monophyletic phylogenetic clades. Comparison of evolutionary divergence scenarios among the four genetic clusters using approximate Bayesian computation indicated that the two C. pseudonaviculata genetic clusters currently found in the United States were derived from different sources, one from the first genetic cluster found in Europe and the second from an unidentified population. Evidence for multiple introductions of this pathogen into the United States and intercontinental migration indicates that future introductions are likely to occur and should be considered in plant disease quarantine regulation.