Determining how Phytophthora ramorum is dispersed across the landscape is critical for understanding the ecology and epidemiology of this influential forest pathogen. To date, most research has focused on abiotic factors, such as rain-splash or wind-blown rain, which most likely contribute to relatively short-distance dispersal. Other abiotic events, such as major storms and down-stream transport of inoculum, may also lead to long-distance dispersal of the pathogen. In contrast, remarkably little research has focused on the potential for biotic dispersal of P. ramorum, although research on other pathogens suggests that it can play an important role.

To address the importance of pathogen dispersal by humans and other vertebrates, Hall Cushman and Ross Meentemeyer — along with research technician Shelly Benson and graduate student Michelle Cooper — are conducting research that explores five questions:
1) Do humans act as dispersal agents of P. ramorum in forested landscapes and do different forms of activity — such as hiking and mountain biking — vary in their effectiveness at dispersing the pathogen? 2) How long can P. ramorum remain viable in soil on bicycle tires and on the soles of hikers' shoes after visiting infected recreational areas? 3) Do birds, deer and squirrels disperse P. ramorum on their feet and are they capable of moving the pathogen horizontally throughout the landscape and vertically into the canopies of terminal and foliar hosts? 4) How far do humans and deer disperse P. ramorum? and 5) Are foliar and terminal hosts more likely to be infected by P. ramorum in areas heavily frequented by humans?

Till now, 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.

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.

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.

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. 

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. 

Cushman, J.H. and Meentemeyer, R.K. 2008. Multi-scale patterns of human activity and the incidence of an exotic forest pathogen. Journal of Ecology 96: 766-776. (PDF)

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