Phytophthora ramorum is an invasive plant pathogen causing considerable and widespread damage in nurseries, gardens and natural woodland ecosystems of the USA and Europe and is classified as a Category 1 pest in Australia. It is of particular interest to Australian plant biosecurity as, like Phytophthora cinnamomi, it has the potential to become a major economic and ecological threat in areas with susceptible hosts and conducive climate. At least three Australian host species have been discovered so far, Eucalyptus haemastoma, Eucalyptus gunnii and Pittosporum undulatum. Asymptomatic sporulation has been confirmed recently for a number of New Zealand species, including a species of Leptospermum, a genus that is widespread throughout Australia within areas of climatic suitability for P. ramorum. Results will be presented of research undertaken in the USA to assess pathogenicity of P. ramorum on Australian native plants and to determine sporulation and survival potential on both symptomatic and asymptomatic tissues. Test species have been selected and sourced from established gardens and nurseries based upon their provenance in regard to areas of climatic suitability for P. ramorum, their relation to known hosts of P. ramorum and their ecological and economical importance to Australian plant industries. Prior screening for the presence of Phytophthora species in the soil profile, and on foliage of selected test species is being conducted. Susceptibility is to be tested using unwounded, detached plant materials, and then scaled up to natural infection studies. Presence and abundance of reproductive structures, sporangia and chlamydospores, will be assessed for all hosts.