Phytophthora pseudosyringae is a self-fertile pathogen of woody plants, particularly associated with tree species from the genera Fagus, Notholithocarpus, Nothofagus and Quercus, which is found across Europe and in parts of North America and Chile. It can behave as a soil pathogen infecting roots and the stem collar region, as well as an aerial pathogen infecting leaves, twigs and stem barks, causing particular damage in the United Kingdom and western North America. The population structure, migration and potential outcrossing of a worldwide collection of isolates were investigated using genotyping-by-sequencing. Coalescent-based migration analysis revealed that the North American population originated from Europe. Historical gene flow has occurred between the continents in both directions to some extent, yet contemporary migration is overwhelmingly from Europe to North America. Two broad population clusters dominate the global population of the pathogen, with a subgroup derived from one of the main clusters found only in western North America. Index of association and network analyses indicate an influential level of outcrossing has occurred in this preferentially inbreeding, homothallic oomycete. Outcrossing between the two main population clusters has created distinct subgroups of admixed individuals that are, however, less common than the main population clusters. Differences in life history traits between the two main population clusters should be further investigated together with virulence and host range tests to evaluate the risk each population poses to natural environments worldwide.

Agrobioinstitute Agricultural Academy Sofia Bulgaria

Department of Agricultural and Forest Sciences and Engineering University of Lleida Lleida Spain

Department of Agricultural Sciences University of Sassari Sassari Italy

Department of Botany and Plant Pathology Oregon State University Corvallis Oregon USA

Department of Cell and Molecular Biology Uppsala University Uppsala Sweden

Department of Forest Engineering Resources and Management Oregon State University Corvallis Oregon USA

Department of Forest Protection and Wildlife Management Mendel University in Brno Brno Czech Republic

Department of Plant Pathology UC Davis Davis California USA

Division of Biotechnology and Plant Health Norwegian Institute of Bioeconomy Research Ås Norway

Flanders Research Institute for Agriculture Fisheries and Food Plant Sciences Unit Merelbeke Belgium

Forest Research Alice Holt Lodge Farnham UK

Joint Research Unit CTFC AGROTECNIO CERCA Lleida Spain

National Bioinformatics Infrastructure Sweden Science for Life Laboratory Sweden

USDA ARS Mycology and Nematology Genetic Diversity and Biology Laboratory Beltsville Maryland USA

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