Spatial Analysis of Risks Posed by Root Rot Pathogen, Phytophthora cinnamomi: Implications for Disease Management DAVID A. KEITH1,2, KEITH L. MCDOUGALL1,3, CHRISTOPHER C. SIMPSON1 AND JILLIAN L. WALSH1 1 NSW Offi ce of Environment & Heritage, PO Box 1967, Hurstville NSW 2220. 2 Australian Wetlands and Rivers Centre, University of New South Wales, Sydney NSW 2052. 3 Department of Environmental Management & Ecology, La Trobe University, PO Box 821, Wodonga, Victoria 3689. Published on 3 September 2012 at http://escholarship.library.usyd.edu.au/journals/index.php/LIN Keith, D.A., McDougall, K.L., Simpson, C.C. and Walsh, J.L. (2012). Spatial analysis of risks posed by root rot pathogen, Phytophthora cinnamomi: implications for disease management. Proceedings of the Linnean Society of New South Wales 134, B147-B179. Phytophthora cinnamomi, a soil-borne pathogen that infects the roots of plants, is listed as a Key Threatening Process under Commonwealth and NSW state biodiversity legislation due to its deleterious effects on native fl ora. In warm temperate eastern Australia, the disease may cause insidious declines in plant species that have slow rates of population turnover, and thereby threaten their long term persistence. Phytophthora cinnamomi has been known to occur in Royal National Park since the 1970s and systematic surveys for the pathogen were carried out a decade ago. Development of effective management strategies to mitigate the impacts of the disease requires information on the spatial distribution of risks posed by the disease. In this study, we use limited disease survey data to identify areas that are most at risk. We propose and apply a simple risk model in which risks of disease impact are proportional to the product of habitat suitability for the pathogen and abundance of susceptible biota. We modelled habitat suitability of the pathogen from available survey data and found that soil landscapes and topographic variables were the strongest predictors. Susceptible fl ora were concentrated on sandstone plateaus. Disease risks were greatest on the sandstone plateaus and lowest in the shale gullies with intermediate levels of risk on shale ridges and the coastal sand plain. The outcomes of this spatially explicit risk assessment will help inform the development of management strategies and priorities for the disease in the Park. Our approach lends itself to broader application to conservation planning in other landscapes and to other threats to biodiversity. Manuscript received 11 January 2012, accepted for publication 2 May 2012. KEYWORDS: disease susceptibility, conservation planning, risk assessment, risk mapping, Royal National Park, species distribution model. INTRODUCTION and Xanthorrhoeaceae are especially susceptible (McDougall 2006; Figure 1). The threat from P. Phytophthora cinnamomi Rands is a soil-borne cinnamomi is recognised in its listing as a key plant pathogen with a very broad host range. It is threatening process nationally under the Environment damaging to crops and native vegetation in many Protection and Biodiversity Act 1999 and in NSW countries. Genetic evidence suggests that it is a recent under the Threatened Species Conservation Act introduction to Australia (Dobrowolski et al. 2003) 1995. but has been present in most States since at least the Phytophthora cinnamomi spreads by motile early 20th Century (Cahill et al. 2008). spores in moist soil over short distances or by Since the association between infection by P. vegetative growth between roots. Spores also enable cinnamomi and death in native plants in Australia was long distance dispersal in storm run off and creeks, established in the late 1960s (Podger 1968), severe or in soil attached to cars and boots. For this reason, damage in native vegetation has been documented in it is often found beside roads, tracks and other places all Australian States (Cahill et al. 2008). Many plants of soil-disturbance, and waterways. Infection occurs in the families Ericaceae, Fabaceae, Proteaceae when motile spores germinate on or near plant roots MANAGEMENT OF ROOT ROT DISEASE Figure 1. Sporadic mortality of susceptible plant species (e.g. Banksia marginata) in spring is charac- teristic of root rot disease caused by Phytophthora cinnamomi in Royal National Park. or stem bases. These produce vegetative strands highly susceptible species, the impact will commonly (mycelia) that grow in host tissue, destroying water involve major structural change with fl ow-on effects conducting cells. Infected plants therefore typically to habitat-dependent plants and animals (McDougall display drought-like symptoms. Death may occur et al. 2005). Importantly, once introduced to a site where a host is especially susceptible to infection or that favours its life cycle, P. cinnamomi is likely to where other stresses are operating on the host (e.g. persist because of the presence of tolerant hosts and drought or insect attack). The mycelia may then the production of resting spores. Recolonisation of produce sporangia (the structure that releases motile infested sites by susceptible hosts may occur and has spores) or, under unfavourable conditions, a resting been recorded (e.g. Weste et al. 1999; McDougall and spore, which is resistant to drying (Cahill et al. 2008). Summerell 2003) but these colonists will be highly Importantly, P. cinnamomi is not dispersed in the air. vulnerable to further infection and their likelihood Host plants vary in their susceptibility, ranging of long-term survival is low. P. cinnamomi is putting from tolerant (where there are no visible symptoms) many species at risk of extinction (Cahill et al. to highly susceptible (where plants are rapidly killed). 2008). Hosts may be tolerant in some habitats but not others, The susceptibility to infection is known for very or only under some climatic conditions (Shearer & few native Australian species (McDougall 2006) Dillon 2006). The overall impact of P. cinnamomi on but there are taxonomic patterns that can allow the native vegetation is determined by local environmental susceptibility of other species to be inferred (Cahill conditions, host availability and time since infestation et al. 2008). For instance, all species of Andersonia (Cahill et al. 2008). For instance, P. cinnamomi can be (family Ericaceae) in Western Australia, for which present and have no visual impact if hosts are tolerant, susceptibility is known, are highly susceptible (Wills can have an impact in one area but not another, despite and Keighery 1994), and species of Xanthorrhoea similar susceptible hosts, if environmental conditions (family Xanthorrhoeaceae) tend to be moderately to are not conducive to spread and infection, or the highly susceptible (McDougall 2006). Curiously, all impact can appear minimal long after infestation if Western Australian species of Banksia are moderately susceptible hosts disappear. At sites dominated by to highly susceptible whereas few eastern Australian B148 Proc. Linn. Soc. N.S.W., 134, 2012 D.A. KEITH, K.L. McDOUGALL, C.C. SIMPSON AND J.L. WALSH Banksia species are highly susceptible (McCredie et (Wilson et al. 2003). An understanding of the spatial al. 1985). However, almost all Banksia species are pattern of risks posed to biodiversity by P. cinnamomi susceptible to infection to some extent (Figure 1). will help direct threat abatement actions to areas Extrapolation of susceptibility will not always be where they address the greatest risks. reliable within genera and families but offers a means In this paper we illustrate a method for mapping of assessing likely vulnerability of communities or the spatial pattern of risks by combining a habitat local fl oras to P. cinnamomi. suitability model for the pathogen with a map Site environmental conditions can greatly showing the abundance and diversity of susceptible infl uence the impact of P. cinnamomi (Wilson et al. fl ora. Our study area includes Royal and Heathcote 2000; Cahill et al. 2008). It is able to complete its National Parks, Garrawarra State Conservation Area life cycle under a great range of temperature (mean and adjacent areas, approximately 30 km south of annual temperature > 7°C) and rainfall (> 400 mm / Sydney city (Figure 2). We fi rst developed a habitat annum) conditions. Some soil types, however, have suitability model for the pathogen using data for P. been found to be less favourable to P. cinnamomi cinnamomi presence obtained by Walsh et al. (2006) activity (e.g. those with antagonistic micro-organisms to relate the occurrence of infested soil samples to (Halsall 1982), high fertility (Shearer and Crane a set of spatially explicit environmental predictor 2003), or high pH / high calcium content (Shearer variables. We then produced a distribution map of and Hill 1989)). Putative relationships have also susceptible fl ora by combining vegetation survey been reported between the incidence of P. cinnamomi data with susceptibility ranks for each species and and soil texture, as well as topographic features that weighting species association with mapped vegetation infl uence soil moisture (Wilson et al. 2000; 2003). types using their frequencies of occurrence. Finally, There is no effective, long-term treatment for P. we combined habitat suitability for the pathogen with cinnamomi once it enters an ecosystem. A chemical susceptibility of fl ora to produce a Phytophthora risk (phosphonate) increases plant resistance but can map for the study area - we used this to identify areas be phytotoxic and needs frequent re-application. where