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Differences Between Genotypes of Phytophthora Ramorum In The UK in Relation to Risk and Disease Management - TH0105

Phytophthora ramorum is a recently introduced plant pathogen that can kill trees and incite disease on a wide range of species including plants in the hardy ornamental nursery trade. It is now accepted that infection of certain plants such as rhododendron results in leaf blight or shoot dieback, typically resulting in abundant sporulation. These so-called ‘foliar hosts’ drive epidemics as the inoculum generated often leads to cankering bark infections on branches/stems of susceptible trees causing in dieback and mortality. Thus managing P. ramorum has focussed on reducing inoculum quantities to epidemiologically insignificant levels by removal of sporulating hosts. Over the past three years, however, two developments have occurred. (1) P. ramorum has transferred to larch (mainly Larix kaempferi) causing a landscape-level epidemic in which larch is a sporulating host and also suffers stem infections with lethal effect. (2) Genetic analysis of P. ramorum has revealed that apart from the European lineage of P. ramorum which is now widespread (termed EU1), a second European lineage (designated as EU2) exists in Northern Ireland and south west Scotland causing disease of larch. Microsatellite analysis of P. ramorum EU1 populations across Europe has also identified dominant genotypes that are unique to the UK and which infect larch and other susceptible plant species.This short project will establish if the EU2 lineage and key EU1 genotypes of P. ramorum represent new and different risks to trees and therefore require different management actions. It has four integrated work streams:

• Determine if the geographical distribution of EU2 P. ramorum and therefore any potentially altered risk extends beyond the two regions currently identified. Isolates of P. ramorum already in culture collections at FR, Fera and AFBI will form the basis of this analysis, and define areas where both lineages overlap as well as the UK-wide distribution of EU2.

• Evaluate fitness characters of EU2 isolates to establish if it differs significantly from the resident EU1 lineage, particularly in relation to larch. Comparisons will be based on 10-15 isolates of both EU1 and EU2 from the UK and measure in vitro temperature-growth response, ability to colonise larch bark and infection and sporulation capacity on larch foliage as indicators of fitness.

• Analyse the pathogenic potential of a smaller sample of EU1 and EU2 isolates against key tree species (including beech, oak, larch, Douglas fir, noble fir and Sitka spruce), to determine if these lineages show adaptive differences to a range of host species.

• Compare temperature-growth responses and sporulation capacity of key genotypes of P. ramorum from the UK. This experimental approach will reveal any differences in pathogen behaviour in relation both to tree hosts and potential for spread in the natural environment.The final report of this work will inform risk-based disease management by defining any changed risk from P. ramorum presented by the EU2 lineage and emerging new genotypes. This is highly relevant to government and policy end-users involved in management of P. ramorum as well plant health regulators. The work will also underpin guidance to UK land managers, to assist understanding of the comparative risks to trees and modifications in disease management that may be needed due to changes in P. ramorum.
Time-Scale and Cost
From: 2012

To: 2013

Cost: £98,435
Contractor / Funded Organisations
Forest Research Agency, Agri-Food & Biosciences Institute, F E R A (FERA)
Plant health              
Plant Pests and Diseases