No abstract

Boxwood blight is an emerging disease of ornamental and native boxwood plants in the family Buxaceae. First documented in the 1990s at a single location in England, the disease is now reported throughout Europe, Asia, New Zealand, and North America. To address the growing concern over boxwood blight, ongoing research focuses on multiple biological and genetic aspects of the causal pathogens and susceptible host plants. Characterization of genetic variation among the Calonectria fungi that cause boxwood blight shows that two unique sister species with different geographic distributions incite the disease. Studies of the pathogen life cycle show the formation of long-lived survival structures and that host infection is dependent on inoculum density, temperature, and humidity. Host range investigations detail high levels of susceptibility among boxwood as well as the potential for asymptomatic boxwood infection and for other plants in the family Buxaceae to serve as additional hosts. Multiple DNA-based diagnostic assays are available, ranging from probe-based quantitative PCR assays to the use of comparative genomics to develop robust diagnostic markers or provide whole genome-scale identifications. Though many questions remain, the research that continues to address boxwood blight demonstrates the importance of applying a multidisciplinary approach to understand and control emerging plant diseases.

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 if the primary pathogen species Calonectria pseudonaviculata (Cps) has remained genetically isolated from its European-limited sister species C. henricotiae (Che), while diversifying into clonal lineages that have migrated among continents. Whole genome sequencing identified 1608 single nucleotide polymorphisms (SNPs) in 67 Cps isolates from four continents and 1017 SNPs in 13 Che 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 Cps 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 the two Cps 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 US and intercontinental migration indicates that future introductions are likely to occur and should be considered in plant disease quarantine regulation.