Genetic Structure and Migration Routes of <i>Phytophthora ramorum</i> into Ireland and Northern Ireland

<i>Phytophthora ramorum</i> is a plant pathogen that causes disease in nurseries and forests in the United States, Canada, Asia, South America, and Europe. Since 2002, disease caused by <i>P. ramorum</i> has led to the destruction of horticultural plants and trees in Ireland (IE) and Northern Ireland (NI). Two lineages, EU1 and EU2, have been reported in both countries; EU1 is more prevalent in IE, while EU2 is more prevalent in NI. We examined the genetic structure of <i>P. ramorum</i> populations in IE and NI from 2005-2022 using microsatellite genotyping. Populations were sorted by country, county, lineage, year, host, and source (nursery or forest). Twenty-nine EU1 multilocus genotypes (MLGs) and six EU2 MLGs were observed. The EU1MG1, the most prevalent MLG in both countries (64% IE, 57% NI), was first observed in nurseries on rhododendron and later found in forests on larch. The EU2MG1 was more common in NI, while EU2MG3 was more common in IE. By 2006, four EU1 MLGs were identified from nurseries and forests in IE suggesting <i>P. ramorum</i> had already been introduced multiple times, likely via the plant trade. Minimum spanning networks revealed similar EU1 MLGs in IE nurseries and forests, and in forests in IE, the UK, and Europe. A T-BAS SSR interactive phylogenetic tool was created to query the EU1 and EU2 lineages. Our data place IE and NI <i>P. ramorum</i> populations into a broader European context and revealed novel genotypes of <i>P. ramorum</i> EU1 and EU2 in IE and NI.