In-vitro characterisation of the four lineages of <Emphasis Type="Italic">Phytophthora ramorum</Emphasis>

Phytophthora ramorum is a plant pathogen with a wide host range including many ornamental hosts and tree species. In Ireland and the UK P. ramorum is known to cause sudden larch death. There are four distinct genetic lineages of P. ramorum, with the fourth lineage (EU2) described in 2012 and only present in Northern Ireland and Scotland. In this work, experiments that compare all four lineages of P. ramorum using several phenotypic characters are described. A total of 166 isolates (EU1: 116, EU2: 40, NA1: 8, NA2: 2) from several EU countries and the United States and several hosts were amassed, and a selection of isolates were compared according to standard phenotypic tests. The EU1 and EU2 isolates tested were all A1 mating type. Regarding linear growth rate, we found the isolates ranked as follows EU2 > NA2 > EU1 > NA1, with all lineages growing fastest at 20 °C. The lineages ranked as NA2 > EU1 > EU2 > NA1 based on their in-vitro aggressiveness on detached wounded Rhododendron leaves, all lineages most aggressive at 20 °C. At 20 °C, we found that there was no significant difference between the EU1 and EU2 lineage based on their linear growth rate or in-vitro aggressiveness. Temperature, host ramet and P. ramorum lineage all had statistically significant effects on the observed aggressiveness of the isolates. From an experimental point of view, our results are broadly in agreement with other phenotypic studies of P. ramorum, finding variation between the lineages, but also variation within the lineages. From an applied perspective, our work on Rhododendron indicates that the EU1 and EU2 lineages pose similar levels of threat to plant health in Ireland and the UK, however, how these results transfer to other hosts (e.g. Larix kaempferi) needs more study.