Changes in the population structure and sporulation behaviour of Phytophthora ramorum associated with the epidemic on Larix (larch) in Britain

During a decade of invasion, the exotic pathogen Phytophthora ramorum has undergone an unexpected change in behaviour during spread into the woodlands and forests of Great Britain. From 2002 to 2008 most outbreaks centred on nurseries and managed gardens with affected hosts almost exclusively broadleaf shrubs and trees. However 2009 saw a major shift as larch tree plantations (Larix) were affected by widespread infection and mortality incited by P. ramorum. To understand the processes underlying the host jump to larch, isolates of the EU1 lineage of P. ramorum collected from 2002 to 2012 were investigated using seven polymorphic microsatellite markers. Analysis of 347 isolates resolved 51 multilocus genotypes (MLGs) which partitioned into two distinct clusters. One comprised MLGs unique to Britain and unknown elsewhere in Europe, the other cluster was primarily of MLGs already known in other European countries but dominated by one genotype, EU1MG1. Pre-2009 isolates were predominantly of the unique British cluster with only a few typical of the European cluster. This reversed after 2009 with European MLGs, especially EU1MG1, becoming increasingly common as the larch epidemic expanded. We hypothesise that the growing dominance of EU1MLG1 has been an important driver in the emergence of the epidemic on larch, aided by its ability to sporulate more abundantly compared with the dominant unique British MLG. European MLGs appear closely associated with the distribution of larch along the west coast of Britain whereas unique British MLGs tend to be concentrated in south west England. The two population clusters suggest at least two separate introductions of the EU1 lineage into Britain with subsequent diversification.