Gene × environment tests discriminate the new EU 2 evolutionary lineage of P hytophthora ramorum and indicate that it is adaptively different

Summary A new evolutionary lineage of the destructive introduced tree pathogen P hytophthora ramorum, EU 2 lineage, was recently discovered attacking larch and other hosts in Northern Ireland and south west S cotland, UK . Sixteen ‘medium × agar concentration × incubation temperature’ stress environments were tested to find a rapid and repeatable method to discriminate the known EU 2 lineage from the EU 1, NA 1 and NA 2 lineages in culture, in particular from the EU 1 already prevalent across the UK ; and to investigate whether EU 2 might be adaptively different. At 25°C on both Carrot agar and V 8 juice agar, the mean radial growth rates of all four lineages were significantly different, with NA 2 >  EU 2 >  EU 1 >  NA 1. At this temperature, EU 2 colonies were not only phenotypically distinct from EU 1 and all other lineages but on average grew three times as fast as EU 1. This indicates that EU 2 is adaptively different from EU 1. Twelve days growth in the environment ‘ V 8 A /2% agar/25°C gave excellent discrimination of all four lineages in three repeat experiments, including clear discrimination of EU 2 from EU 1. Each lineage exhibited a distinctive colony pattern. The utility of this test environment was examined further by screening fresh UK isolates of unknown lineage from new larch outbreak sites alongside standard isolates. The lineage assignments predicted were corroborated by gene sequencing and RFLP profiling. These results also revealed that the EU 2 lineage was present at several new larch sites in south west S cotland, whereas isolates from geographically adjacent areas such as the I sle of M ull, north west S cotland, the I sle of M an and north west E ngland were all of EU 1 lineage.