Gene × environment tests discriminate the new EU2 evolutionary lineage of Phytophthora ramorum and indicate that it is adaptively different

Abstract

A new evolutionary lineage of the destructive introduced tree pathogen Phytophthora ramorum, EU2 lineage, was recently discovered attacking larch and other hosts in Northern Ireland and south west Scotland, UK. Sixteen 'medium 9 agar concentration 9 incubation temperature' stress environments were tested to find a rapid and repeatable method to discriminate the known EU2 lineage from the EU1, NA1 and NA2 lineages in culture, in particular from the EU1 already prevalent across the UK; and to investigate whether EU2 might be adaptively different. At 25°C † on both Carrot agar and V8 juice agar, the mean radial growth rates of all four lineages were significantly different, with NA2 > EU2 > EU1 > NA1. At this temperature, EU2 colonies were not only phenotypically distinct from EU1 and all other lineages but on average grew three times as fast as EU1. This indicates that EU2 is adaptively different from EU1. Twelve days growth in the environment 'V8A/2% agar/25°C † gave excellent discrimination of all four lineages in three repeat experiments, including clear discrimination of EU2 from EU1. 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 EU2 lineage was present at several new larch sites in south west Scotland, whereas isolates from geographically adjacent areas such as the Isle of Mull, north west Scotland, the Isle of Man and north west England were all of EU1 lineage.

In planta detection used to define the distribution of the European lineages of Phytophthora ramorum on larch (Larix) in the UK

“…Previous studies reported the EU2 lineage from Japanese larch, bilberry, rhododendron and non‐native red oak (Van Poucke et al ., ; Franceschini et al ., ). The present study has provided the first confirmed findings of natural infection by EU2 on European larch, hybrid larch, beech, noble fir and western hemlock.…”

“…Overall, the distribution of the EU2 lineage is similar to that reported previously, and there is no evidence of rapid invasive spread into other parts of Scotland or expansion into either England or Wales. However, EU2 is more widespread in southwest Scotland than previously reported (Van Poucke et al ., ; Franceschini et al ., ) and its range now includes more southern and eastern parts of the Dumfries and Galloway region. In 2013 heavy larch mortality occurred in this part of southwest Scotland, with around 4000–6000 ha of larch (out of 8000–9000 ha in total) estimated to be affected following aerial surveys.…”

“…Such a scenario is supported by the study of Franceschini et al . () who showed that EU2 isolates grew faster and tolerated higher temperatures than those of the EU1 lineage. Moreover, comparative experiments have shown that the EU2 lineage is more aggressive than EU1 at colonizing larch bark tissue (Harris et al ., ; Webber et al ., ) and is therefore likely to kill affected trees more rapidly.…”

“…It can be distinguished from the other lineages (EU1, NA1 and NA2) both genetically (e.g. using microsatellite markers or multilocus sequence analysis) and phenotypically (based on colony morphology and temperature responses) (Van Poucke et al, 2012;Franceschini et al, 2014). The consequences of EU2 arrival in the UK are under investigation but so far it has been found infecting Japanese larch, non-native red oak (Quercus rubra), bilberry (Vaccinium myrtillus) and rhododendron.…”

An evolutionary ecology perspective to address forest pathology challenges of today and tomorrow

“…The idea that "nothing in forest pathology makes sense except in the light of evolution," paraphrasing Dobzhansky, could now seem obvious and not deserving of further development (Varki 2012). Many recent articles dealing with forest pathology indeed have an evolutionary background (e.g., Hansen and Goheen 2000;Pinon and Frey 2005;Kinloch et al 2008;Garbelotto et al 2010;Hayden et al 2011;Cruikshank and Jaquish 2014;Franceschini et al 2014). However, we contend that an evolutionary perspective is still not sufficiently emphasized and applied to the management of forest diseases.…”