Sudden Oak Death (SOD), caused by the fungal-like pathogen Phytophthora ramorum, is the latest in a long line of exotic forest diseases that includes chestnut blight and Dutch elm disease. Potentially millions of tanoak and oak trees in coastal forests of California and Oregon have been lost to SOD over the past 10 years. This project will examine the environmental and biological circumstances that initially led to the emergence of P. ramorum and the subsequent disease-related changes to the forest environment. We will use a combination of field, greenhouse, and laboratory experiments along with geographical information system (GIS) and mathematical modeling approaches to research the spatial and temporal dynamics of SOD. The project will examine how human-induced changes in landscape structure and composition of forests when combined with weather patterns (e.g., El Nino) may have influenced the establishment and spread of pathogen in California forests. Following P. ramorum invasion, changes may also occur to the pathogen, host and forest environment. P. ramorum is a generalist pathogen that infects plants in over 45 genera including ferns, gymnosperms, monocots and dicots. SOD epidemics in California forests are primarily driven by the presence of these associated host species that serve as sources of the pathogen, and not by the oaks themselves. Because mortality is often restricted to oak and tanoak, the broad host range of P. ramorum will allow us to test hypotheses of plant competition mediated by a pathogen. The broad host range of the pathogen may also allow for P. ramorum populations to evolve towards increased virulence and/or increased diversity. However, over time we would also expect that invasion by P. ramorum will influence the occurrence and spatial distribution of resistant and tolerant host genotypes. Finally, the role of parasites in influencing ecosystem functioning (e.g., nutrient cycling) has often been overlooked. In areas where P. ramorum-associated overstory mortality has significantly impacted composition of coastal forests, we will analyze changes in forest floor inputs, organic matter, decomposition rates, and nitrogen dynamics. A better understanding of invasion processes and impacts of generalist pathogens is critical for developing management and regulatory strategies to protect natural ecosystems. This research will have a number of broad impacts in disease policy and management, public outreach, and education. A better understanding of invasion processes and impacts of generalist pathogens is critical for developing management and regulatory strategies to protect natural ecosystems. The PIs are actively involved with advising state, Federal (APHIS) and International committees on SOD and other forest diseases. P. ramorum has the broadest host range of any fungal pathogen ever attempted to be quarantined and will serve as a model for responses to future pathogen introductions. The project will also support the education of 4 graduate students, 3 post-docs as well as a number of undergraduate students. Each lab has a history of including under-represented groups. As part of their education, post-docs and graduate students will be involved in public outreach efforts. Pre-existing collaborations on research and management of SOD will be continued with several Native American tribes (Pomo, Hoopa, and Yurok) in Northern California.
