Multi-scale patterns of human activity and the incidence of an exotic forest pathogen

1. Plant pathogens can have major impacts on diverse taxa and ecological systems world-wide, and some of the most conspicuous of these are invasive non-native species. Although many factors are known to influence the distribution and abundance of plant pathogens, the extent to which humans play a role is less well-known but still critical for understanding the dynamics of invasive pathogens in natural systems. 2. One invasive pathogen of great concern world-wide is Phytophthora ramorum, which causes Sudden Oak Death, an emerging forest disease. Here, we examined the influence of humans and a range of environmental factors on the distribution of P. ramorum at three distinct spatial scales in northern and central California. 3. At the local scale, P. ramorum more commonly occurred in soil on hiking trails used heavily by humans than in soil from adjacent areas off trails. These results support the hypothesis that humans dispersed the pathogen within already infected areas and into areas lacking local sources of inoculum. 4. At the landscape scale, using a network of 202 randomly located plots across a 275 km(2) area, we found that forests on public land open to recreation exhibited higher prevalence of disease in a critical infectious host tree (bay laurel, Umbellularia californica), than forests on private lands. Infection levels were also higher in plots surrounded by large amounts of forest with warm climatic conditions and greater potential soil moisture. Although prevalence of diseased canker hosts (Quercus agrifolia, Q. kelloggii and Lithocarpus densiflora) was positively associated with dominance of bay laurel and rainy season temperatures, it was not significantly related to public access, and we hypothesize that this occurred because our study area was in the early stages of infection. 5. At the regional scale, the probability of disease occurrence at 165 sites distributed across the geographic range of P. ramorum in California increased significantly as human population density increased in the surrounding area. Chances of infection also increased significantly with precipitation and presence of bay laurel. 6. Synthesis. Our results suggest that human activity - along with temperature, moisture and host composition - is associated with increased prevalence of an influential exotic forest pathogen. These results indicate that there may be conflicts between humans and disease, and that efforts to address this issue may require aggressive management of human activity.