UNDERSTANDING HOST JUMPS: EVALUATING THE RISK OF PHYTOPHTORA PLUVIALIS TO WESTERN HEMLOCK IN THE USA

Global trade and climate change accelerate and compound the emergence of plant pathogens and impacts on natural and managed ecosystems. Phytophthora pluvialis is a foliar pathogen of conifer trees that has repeatedly emerged in the United States (US) and invaded New Zealand (NZ) and more recently the United Kingdom (UK). Its emergence has caused severe epidemics on economically important conifer trees in both plantations and wildland forests. In its most recent emergence, P. pluvialis not only jumped to a new host but was associated with a devastating shift in disease etiology. In addition to infecting foliage, the pathogen infected the main stem of trees and caused cankers that led to tree mortality (Fig. 1). This change in disease etiology is of particular concern because of potential effects on the forest industry in the Pacific Northwest of the US. In this region western hemlock and Douglas-fir, the two main hosts of this pathogen, make up most of the crop trees planted on the 8.1 million acres of private industrial forest land. Testing the following specific hypotheses is essential for determining impacts of emergence by P. pluvialis on the US forest industrySpecific hypothesis 1: The invasion in the UK by P. pluvialis is the result of a genetic bottleneck of the host followed by accidental selection of susceptible western hemlock genotypes.Specific hypothesis 2: The shift in disease etiology is the result of emergence of a new clade of P. pluvialis able to infect hemlock and cause stem cankers.Specific hypothesis 3: Differences in environmental conditions among locations explain changes in the etiology of the disease and host shifts by P. pluvialis.The Objectives of this proposal are:Characterize the variation in susceptibility to P. pluvialis within and among populations of Douglas-fir and western hemlock.Genotype western hemlock and Douglas-fir populations from the US and the UK.Evaluate differences in susceptibility of leaves and stem tissue among populations of western hemlock and Douglas-fir.Evaluate the epidemiological risk to western hemlock and Douglas-fir tree farms in the USA.Evaluate growth and sporulation potential of representative P. pluvialis isolates across a range of temperatures and on inoculated seedlings.Contrast infection biology on Douglas-fir and western hemlock.Develop and deploy a multiplex Sherlock assay to detect major lineages of P. pluvialis.Validation of the SHERLOCK assay quantitation of sporulation.