Jarrah dieback caused by Phytophthora cinnamomi is a major factor affecting the ecology and management of the jarrah (Eucalyptus marginata) forest. The disease is an exeptional example of an introduced pathogen with a wide host range causing great damage to a diverse but mainly susceptible plant community. This review integrates the information relevant to the management of the jarrah forest in the presence of disease. Although P. cinnamomi is the Phytophthora species most frequently isolated from areas of dying vegetation, P. citricola, P. cryptogea, P. megasperma var. sojae, P. nicotianae and unnamed Phytophthora species have been recovered from the jarrah forest. All of these Phytophthora species have a wide host range and a dependency on moisture to complete their lift cycles. Phytophthora cinnamomi is distributed throughout south-western Australia, but affected areas of forest are most frequent on the western edge of the Darling Scarp and decrease in number with distance east. There is also a north/south gradient with southern forest less affected than northern forest. The regional pattern of affected areas has not occurred by accident but resulted from the interactions between topography, climate, soil, drainage, vegetation, and intensity of human activity creating niches favourable for the pathogen. The jarrah forest remains on the western edge of the ancient Great Plateau of Western Australia on soils unsuited to agriculture. The climate is typically mediterranean with cool wet winters and hot dry summers. The soils reflect earlier periods of peneplanation, laterization and uplift. The gently undulating remnant plateau uplands are mantled with relict laterite. Red earth deep loams have developed on the slopes of dissected river valleys on the uplifted western edge of the plateau. Duplex soils with a range of textures and depths predominate on gentle slopes near the eastern edge of the forest. Structure, morphology, hydrological properties, temperature, and fertility are the most important soil factors influencing the life cycle of P. cinnamomi in the jarrah forest. Fire is a natural part of the jarrah forest environment. Understorey composition affected by fire intensity and frequency can greatly affect the behaviour of P. cinnamomi. The forest is characteristically dry sclerophyll with a uniform overstorey dominated by jarrah, but with a diverse shrub layer having a high level of endemism and speciation. The understorey occurs as a complex mosaic wherein the vegetation integrates and hence serves as an indicator of environmental conditions. The variation in understorey vegetation is a multidimensional continuum which has been classified into site-vegetation types, each having a characteristic group of indicator species. The vegetation has adapted and specialized over a long geological period in response to isolation by barriers.....
