Abstract
The most important oomycete forest pathogens comprise two genera: Pythium and the formidable genus Phytophthora, whose name appropriately means 'plant destroyer'. Pythium spp. cause seed and root rots and damping off diseases that thwart seedling establishment, and have been implicated in helping to drive forest diversity patterns through increased disease pressures on seedlings closest to their mother tree (Janzen, 1970; Connell, 1971). In contrast, Phytophthora spp. can cause disease at every life stage of forest trees, from root to crown, and from trunk cankers to foliar blights (Erwin and Ribeiro, 1996). They are remarkably flexible and effective pathogens with an unusual genetic architecture that may favour the rapid evolution of pathogenicity (Jiang et al., 2008; Raffaele et al., 2010; Seidl et al., 2011). Outbreaks of disease caused by Phytophthora spp. (especially when they have been introduced to new systems) have been documented with dramatic, and sometimes disastrous, effects since the mid 1800s, when European and North American chestnuts (Castanea spp.) began dramatic declines from chestnut ink disease, a root rot caused in part by the extreme generalist Phytophthora cinnamomi Rands (Crandall et al., 1945; Anagnostakis, 1995). P. cinnamomi is notorious for the massive mortality it has caused in jarrah (Eucalyptus marginata Donn ex Sm.) forests in Western Australia, where it was first observed in the 1920s (Podger, 1972). P. cinnamomi causes root disease in agricultural and forest systems worldwide with varying degrees of virulence, but as Phytophthora dieback it has been seen to kill 50-75% of the species in sites in Western Australia, in some cases leaving every tree and much of the understorey dead (Weste, 2003). Shearer et al. (2004) estimate that of the 5710 described plant species in the South West Botanical Province of Western Australia, approximately 2300 species are susceptible, 800 of which are highly susceptible.