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Long-term surveillance of Phytophthora cinnamomi reveals no evidence of increased detections and new insights for monitoring and management

Tanya Mason , Keith McDougall, Phil Craven, Christopher Simpson, Gordana Popovic, David Keith

Abstract

Context Phytophthora cinnamomi Rands is an introduced soil-borne pathogen that causes population decline in susceptible species and changes native vegetation compositions. Land managers require information about changing disease incidence, predisposing site factors and potential efficiencies in field identification of the disease. Previous sampling for P. cinnamomi in southern Sydney conservation reserves has provided a framework to monitor pathogen incidence. Aims We specifically asked: (1) Is the incidence of P. cinnamomi disease intensifying, decreasing or stable? (2) Is visual assessment of disease symptoms at the community or sentinel species level a good predictor of positive P. cinnamomi laboratory assays? (3) Is the incidence of P. cinnamomi related to access, vegetation type, human visitation or landscape position? Methods We assembled data from historic surveys in Dharawal Reserves in 2008 (n = 26) and 2014 (n = 110) and undertook additional surveys in 2022 (n = 110). We supplemented detections in 2008 with 2006-2008 records from other southern Sydney conservation reserves (n = 147). We used laboratory detection results, field observations of plant symptoms and environmental data to assess trends. Key results We did not find evidence of increased pathogen incidence over time; however, detection was spatially and temporally variable among sites. Proximity to waterways appeared to increase the likelihood of P. cinnamomi presence. We found differences between visual field assessment of disease expression and laboratory assay of pathogen detection. We observed marginal improvement in detection agreement when we subsetted the data to sites that explicitly identified Xanthorrhoea as potential host species. A sample size analysis indicated considerable increase in sampling effort would be required to detect consequential changes in the long-term P. cinnamomi status. Conclusions Landscape-scale monitoring of P. cinnamomi incidence requires greater sampling intensity and duration to support reliable inferences about trends. Locally focused management and monitoring efforts, which consider susceptible sentinel species dynamics, may inform the protection of specific assets at risk from infection by P. cinnamomi.

BT24037  Accepted 27 November 2024

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