Accounting for the potential of oak-savanna decline caused by Phytophthora cinnamomi: conceptual framework

Mediterranean agroforestry systems, dominated by holm oak and cork oak woodlands, constitute ecosystems of high ecological, productive and socio-economic value. However, these systems are currently undergoing global decline as a result of multiple factors, among which climate change and the activity of some diseases, mainly Phytophthora cinnamomi Rands stand out. This soil-borne oomycete causes necrotic lesions in roots and stems, leading to fine-root loss, reduced water uptake, progressive decline, and tree mortality. Its life cycle includes both sexual and asexual phases; during the latter, motile zoospores require free soil water for infection, enabling movement and contact with host roots. In this context, Mediterranean climatic conditions are particularly favorable for pathogen proliferation, characterized by relatively warm and wet winters and springs with frequently waterlogged soils, followed by long, dry summers that induce severe water stress in trees, exacerbate root disease symptoms, and contribute to the decline of these agroforestry systems. Therefore, it is essential to identify areas more prone to be affected by the pathogen and, when affected, their potential to be a source for pathogen spread. Despite the enormous efforts carried out for monitoring and improving the understanding of pathogen propagation, there is still a way forward to better quantify pathogen spread potential. 

In this study, we adapted an existing index-based framework to assess the potential for non-point pollution (PNPI) at the watershed scale to model the spread of Phytophthora. The study has been carried out in Guadalquivir river basin, southern Spain. We assume that the spread of Phytophthora follows the same logic as the one followed by other substances previously modeled using PNPI. Hence, we reinterpret this framework to map the spatial potential for hydrologically mediated pathogen spread, assuming water as the main vector, the importance of landscape connectivity, and enhanced spread potential during wet years. The adapted index integrates three components: (i) a source indicator, accounting for land cover, landscape connectivity, inoculum pressure, and soil characteristics; (ii) a hydrological connectivity indicator, representing the potential for subsurface and downslope transport; and (iii) a runoff generation and transport capacity indicator, describing the landscape ability to mobilize and convey inoculum. Temporal variability in precipitation is incorporated to capture interannual climate control on spread potential. The main methodological challenge was redefining the source indicator to include: (a) host presence and density (oak cover, woodland fraction, fragmentation), (b) inoculum pressure (distance to known infection foci and presence of symptomatic trees), and (c) soil and site suitability (texture, drainage, water retention, and conditions favoring pathogen persistence). 

We prove that using simple mathematical expressions and low data requirements, we produced annual maps of estimated Phytophthora decline potential, providing a spatially explicit screening tool to identify areas with higher potential for hydrologically driven spread, showing correspondence with known infection sources. 

Acknowledgments: This research was performed within DRYAD Project, which has received funding from the European Union’s Horizon Europe research and innovation program under grant agreement 101156076. This work is part of the grant RYC2022-035320-I, funded by MCIN/AEI/10.13039/501100011033 and FSE+