Genetic Structure and Migration Routes of Phytophthora ramorum into Ireland and Northern Ireland

Phytophthora ramorum 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 P. ramorum has led to the destruction of horticultural plants and trees in Ireland and Northern Ireland. Two lineages, EU1 and EU2, have been reported in both countries; EU1 is more prevalent in Ireland, whereas EU2 is more prevalent in Northern Ireland. We examined the genetic structure of P. ramorum populations in Ireland and Northern Ireland from 2005 to 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. EU1MG1, the most prevalent MLG in both countries (64% Ireland, 57% Northern Ireland), was first observed in nurseries on rhododendron and later found in forests on larch. EU2MG1 was more common in Northern Ireland, whereas EU2MG3 was more common in Ireland. By 2006, four EU1 MLGs were identified from nurseries and forests in Ireland, suggesting that P. ramorum had already been introduced multiple times, likely via the plant trade. Minimum spanning networks revealed similar EU1 MLGs in Ireland nurseries and forests and in forests in Ireland, the United Kingdom, and Europe. A T-BAS SSR interactive phylogenetic tool was created to query the EU1 and EU2 lineages. Our data place Ireland and Northern Ireland P. ramorum populations into a broader European context and reveal novel genotypes of P. ramorum EU1 and EU2 in Ireland and Northern Ireland.