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Phytophthora ramorum: Susceptibility of Australian plants, potential geographic range and science into policy and management

Ireland, Kylie (2011) Phytophthora ramorum: Susceptibility of Australian plants, potential geographic range and science into policy and management. PhD thesis, Murdoch University.

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Phytophthora ramorum is an invasive plant pathogen causing considerable and widespread damage in nurseries, gardens and natural woodland ecosystems of the USA and Europe. In Australia, where it is classified as a Category 1 emergency plant pest, it has the potential to become a major economic and ecological threat in areas with susceptible hosts and a favourable climate. Phytophthora ramorum causes three distinct diseases on susceptible plants: ramorum leaf blight, ramorum shoot dieback and sudden oak death (characterised by lethal bole cankers). In some species, foliar infection can play a crucial role in disease development by producing inoculum that can drive an epiphytotic. The geographic origin of P. ramorum remains unknown and an understanding of the environmental requirements most conducive to establishment and persistence of disease under natural conditions is still poorly understood. This study aimed to: (a) provide an understanding of the foliar, branch and bole susceptibility to P. ramorum and sporangia producing potential of a broad range of Australian plant species; (b) develop a climate-based model of the potential geographic range of the pathogen; and (c) analyse the effectiveness of integration of scientific knowledge between European and North American policy and management responses to P. ramorum.

Detached leaves, branches and logs of up to 70 Australian native plant species were tested for their susceptibility and sporulation potential. Foliar susceptibility was tested using detached leaf assays for 70 Australian native plant species; twenty-eight of these species were tested for their ability to produce sporangia on foliage. Branch dieback susceptibility was tested for 66 of these species, six of which were further tested for their susceptibility to bole cankers caused by P. ramorum using a sealed log assay. All materials were sourced from native woodlands, established gardens and arboreta in California. Positive control species known to be naturally highly susceptible to P. ramorum were included in all experiments.

All species tested were capable of being infected by P. ramorum. Highly susceptible foliar hosts included Banksia attenuata, Eucalyptus delegatensis, E. denticulata, E. viminalis, Isopogon cuneatus, I. formosus and Leptospermum scoparium. Hedycarya angustifolia, Olearia argophylla, Phyllocladus aspleniifolius, Pittosporum undulatum and Podocarpus lawrencei were identified as potentially resistant foliar hosts. Putative sporulating hosts include five members of the Myrtaceae: Agonis flexuosa, C. ficifolia, E. haemastoma, E. delegatensis and E. viminalis. Highly susceptible branch hosts included E. denticulata, E. sideroxylon, E. viminalis, Hardenbergia violaceae, I. formosus and N. cunninghamii. Thirteen potentially tolerant branch dieback hosts were identified and included B. attenuata, B. marginata, Billardiera heterophylla, E. haemastoma, E. regnans and P. undulatum. Eucalyptus regnans was identified as a potentially highly susceptible bole canker host, while E. diversicolor and E. viminalis were considered potentially tolerant species to bole cankers caused by P. ramorum.

A simulation model was developed using CLIMEX to estimate the global climate suitability patterns for establishment of P. ramorum. Growth requirements and stress response parameters were derived from ecophysiological laboratory observations and site-level transmission and disease factors related to climate data in the field. Models fitted to the European (EU1) and combined EU1 and North American (NA1) genotypes indicated that European genotypes may be constrained to a greater degree by higher temperatures than North American genotypes. The combined risk model suggests that the invasion of P. ramorum in both North America and Europe is still in its infancy and it is presently occupying a small fraction of its available range. Southern Europe may be at greater risk of invasion should the NA1 genotype be introduced into warmer areas unsuitable for the predominant EU1 genotype. Phytophthora ramorum appears to be climatically suited to large areas of Australasia (including New Zealand), Africa and South America. Potential distribution in Australia indicates south east coastal Australia, the southwest region of Western Australia and Tasmania are at highest risk of invasion.

Comparison of the integration of science into policy-making and control efforts in Europe and North America are varied, representing the use of many different ‘boundary arrangements’. Experiences with P. ramorum in these regions indicate that future biosecurity efforts to prevent the entry and establishment of P. ramorum and other invasive organisms may benefit from: (i) fostering local management approaches which connect and build relationships with affected communities and build capacity accordingly; (ii) incorporating structural arrangements for the integration of science into policy at a national level, encouraging scientists and policy makers to directly engage with one-another to allow for the rapid dissemination of new knowledge directly applicable to policy applications; (iii) aiming to produce regional or global pest risk analyses which enable knowledge and research cost-sharing, and; (iv) investment in studies outlining the effectiveness and success of different boundary arrangements in achieving positive biosecurity outcomes.

These results extend the known potential host range for P. ramorum and define its potential geographic range, confirming it as a potential threat to Australian plant industries and ecosystems. Caution is advised when interpreting these results; the species studied represent only a small proportion of Australian taxa which exist in climatically suitable areas for the pathogen in Australia and invasive organisms may behave differently in novel locations given different environmental and management constraints. Nevertheless, risk predictions generated by the model, an understanding of the pathogen’s potential host range and analysis of the best way to integrate this knowledge into policy and management efforts will allow us to target high risk areas for early detection surveillance and assist Australian regulators in developing appropriate quarantine policies and protocols.

Item Type: Thesis (PhD)
Murdoch Affiliation(s): School of Biological Sciences and Biotechnology
Supervisor(s): Hardy, Giles, Huberli, Daniel, Dell, Bernard and Smith, Ian
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