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Abstract Anthropogenic activities have altered historical disturbance regimes, and understanding the mechanisms by which these shifting perturbations interact is essential to predicting where they may erode ecosystem resilience. Emerging infectious plant diseases, caused by human translocation of nonnative pathogens, can generate ecologically damaging forms of novel biotic disturbance. Further, abiotic disturbances, such as wildfire, may influence the severity and extent of disease‐related perturbations via their effects on the occurrence of hosts, pathogens and microclimates; however, these interactions have rarely been examined.The disease ‘sudden oak death’ (SOD), associated with the introduced pathogenPhytophthora ramorum, causes acute, landscape‐scale tree mortality in California's fire‐prone coastal forests. Here, we examined interactions between wildfire and the biotic disturbance impacts of this emerging infectious disease. Leveraging long‐term datasets that describe wildfire occurrence andP. ramorumdynamics across the Big Sur region, we modelled the influence of recent and historical fires on epidemiological parameters, including pathogen presence, infestation intensity, reinvasion, and host mortality.Past wildfire altered disease dynamics and reduced SOD‐related mortality, indicating a negative interaction between these abiotic and biotic disturbances. Frequently burned forests were less likely to be invaded byP. ramorum, had lower incidence of host infection, and exhibited decreased disease‐related biotic disturbance, which was associated with reduced occurrence and density of epidemiologically significant hosts. Following a recent wildfire, survival of mature bay laurel, a key sporulating host, was the primary driver ofP. ramoruminfestation and reinvasion, but younger, rapidly regenerating host vegetation capable of sporulation did not measurably influence disease dynamics. Notably, the effect ofP. ramoruminfection on host mortality was reduced in recently burned areas, indicating that the loss of tall, mature host canopies may temporarily dampen pathogen transmission and ‘release’ susceptible species from significant inoculum pressure.Synthesis. Cumulatively, our findings indicate that fire history has contributed to heterogeneous patterns of biotic disturbance and disease‐related decline across this landscape, via changes to the both the occurrence of available hosts and the demography of epidemiologically important host populations. These results highlight that human‐altered abiotic disturbances may play a foundational role in structuring infectious disease dynamics, contributing to future outbreak emergence and driving biotic disturbance regimes. 

For nearly two decades, stream baiting of northern and central California coastal streams has been an important tool in the management of sudden oak death, a devastating forest disease caused by the oomycete Phytophthora ramorum. Phytophthora species are baited with floating rhododendron leaves, serving as an early detection tool for the presence of P. ramorum in watersheds across more than 800 km of California coastline. While this long-standing management tool is focused on a single species of Phytophthora, other species of Phytophthora have been baited alongside P. ramorum, and this study documents the presence and distribution of 22 Phytophthora and Nothophytophthora species across the northern and central coasts of California. Although P. ramorum was isolated at the greatest number of sites, several species in subgeneric clade 6 were also abundant and widespread, a common feature of Phytophthora stream baiting studies. Clade 3 species P. nemorosa, P. pluvialis, and P. pseudosyringae were also frequently isolated in northern coastal streams. The species Nothophytophthora caduca and the genus Nothophytophthora are reported for the first time in North America along with the first report of P. pluvialis in California. Two novel species, Nothophytophthora sp. californica and P. sp. aureomontensis (a member of the P. citricola species complex) are provisionally named. Mitochondrial sequences revealed multiple hybridization events between P. lacustris and P. riparia. Stream monitoring can serve as an important tool for monitoring ongoing Phytophthora invasions as well as establishing baseline pathogen communities, critical data for preventing future invasions. 

Many coastal forests stretching from central California to southwest Oregon are threatened or have been impacted by the invasive forest pathogen Phytophthora ramorum, cause of sudden oak death. We analyzed a set of stand-level forest treatments aimed at preventing or mitigating disease impacts on stand composition, biomass, and fuels, using a before-after-control-intervention experiment with a revaluation after five years. We compared the effects of restorative management in invaded stands to preventative treatments in uninvaded forests. The restorative treatments contrasted two approaches to mastication, hand-crew thinning, and thinning with pile burning with untreated controls (N=30) while the preventative treatments were limited to hand-crew thinning (N=10). Half of the restoration treatments had basal sprouts removed two- and four-years after treatment. All treatments significantly reduced stand density and increased average tree size without significantly decreasing total basal area both immediately and five years after treatments. Preventative treatments also significantly increased dominance of timber species not susceptible to P. ramorum. Follow-up basal sprout removal in the restoration experiment appears to maintain treatment benefits to average tree size and may be associated with small decreases in stand density five years after initial treatment. Our study demonstrates that for at least five years, a range of common stand management practices can improve forests threatened or impacted by sudden oak death. 

Purpose of Review I aim to contextualize wildfire-disease interactions with the goal of building a better understanding of where these may be of ecological importance and problems for sustainable forest management. Recent Findings While wildfire-disease interactions have been documented, they are not well represented in the ecological literature, probably because they require considerable effort or serendipity to rigorously quantify. Examples of disease-fire interactions are relatively limited and tend to be clearer in systems where fire and disease are management problems. The most resolved systems include Phytophthora pathogens although wildfire-disease interactions are not limited to these pathogens. Documented interactions encompass a range of effects which include the magnification of problems associated with each disturbance. Wildfire-disease interactions are also likely to shape basic ecological function in systems where both wildfire and disease are common but not necessarily critical management problems. Climate change has altered the fundamental controls on both fire and disease suggesting it will also alter the magnitude and likelihood (occurrence or detection) of disease-fire interactions. Summary I present a framework for linking wildfire-disease interactions and highlight the importance of host community/fuels structure on linking and mediating these interactions. I provide a series of examples where understanding interactive effects, interfacing with climate change, and the magnitude of changes to wildfire and disease intensification are of practical value and/or advance basic ecological knowledge. While much remains to be understood about these interactions, I make the argument that, in some cases, management can address both problems simultaneously. 

A collection of 30 Phytophthora cactorum and 12 P. pseudotsugae (subclade 1a) strains isolated from several recent surveys across California was phylogenetically compared to a worldwide collection of 112 conspecific strains using sequences from three barcoding loci. The surveys baited P. cactorum from soil and water across a wide variety of forested ecosystems with a geographic range of more than 1000 km. Two cosmopolitan lineages were identified within the widespread P. cactorum, one being mainly associated with strawberry production and the other more closely associated with apple orchards, oaks and ornamental trees. Two other well-sampled P. cactorum lineages, including one that dominated Californian restoration outplantings, were only found in the western United States, while a third was only found in Japan. Coastal California forest isolates of both Phytophthora species exhibited considerable diversity, suggesting both may be indigenous to the state. Many isolates with sequence accessions deposited as P. cactorum were determined to be P. hedraiandra and P. ×serendipita, with one hybrid lineage appearing relatively common across Europe and Asia. This study contains the first report of P. pseudotsugae from the state of California and one of the only reports of that species since its original description. 

Sudden oak death (SOD), caused by the generalist pathogen Phytophthora ramorum, has profoundly impacted California coastal ecosystems. SOD has largely been treated as a two-host system, with Umbellularia californica as the most transmissive host, Notholithocarpus densiflorus less so, and remaining species as epidemiologically unimportant. However, this understanding of transmission potential primarily stems from observational field studies rather than direct measurements on the diverse assemblage of plant species. Here, we formally quantify the sporulation potential of common plant species inhabiting SOD-endemic ecosystems on the California coast in the Big Sur region. This study allows us to better understand the pathogen’s basic biology, trajectory of SOD in a changing environment, and how the entire host community contributes to disease risk. Leaves were inoculated in a controlled laboratory environment and assessed for production of sporangia and chlamydospores, the infectious and resistant propagules, respectively. P. ramorum was capable of infecting every species in our study and almost all species produced spores to some extent. Sporangia production was greatest in N. densiflorus and U. californica and the difference was insignificant. Even though other species produced much less, quantities were nonzero. Thus, additional species may play a previously unrecognized role in local transmission. Chlamydospore production was highest in Acer macrophyllum and Ceanothus oliganthus, raising questions about the role they play in pathogen persistence. Lesion size did not consistently correlate with the production of either sporangia or chlamydospores. Overall, we achieved an empirical foundation to better understand how community composition affects transmission of P. ramorum.