Spatial relationship between Phytophthora ramorum and roads or streams in Oregon tanoak forests

• Patterns in the distribution of disease may aid the management of sudden oak death.
• SOD sites did not occur closer to roads than expected by chance.
• Road use has not contributed to long distance dispersal of P. ramorum in Oregon.
• Sites did occur more commonly along stream ways, especially in inland watersheds.
• Dispersal of inoculum may be occurring in turbulent air or stream waters.

The pathogen, Phytophthora ramorum, causal agent of sudden oak death (SOD) of oaks and tanoaks, continues to expand its range within Oregon despite an effort to eradicate it from native forests. With its early detection and prompt removal of infected hosts, the Oregon SOD eradication program has produced a landscape distribution of disease resulting predominantly from the long distance (100 m to 4 km) dispersal of inoculum between sites. Using a regionally restricted randomization test reflecting the south to north intensification of the SOD epidemic in Oregon, we assessed if the movement of P. ramorum between sites was spatially dependant upon roads and streams, topographic features associated with the landscape-scale movement of soil and water borne inoculum of related Phytophthora spp. Dissimilar to other forest Phytophthora spp. we found no association between SOD sites and the road network. We did, however, determine that SOD sites are occurring closer to streams than would be expected by chance, especially in regions with microclimates less conducive to establishment. Environmental conditions and/or dispersal mechanisms associated with streams may contribute to the distribution of SOD in Oregon tanoak forests. Monitoring and management should therefore concentrate on susceptible forests in close proximity to streams, especially in stands further inland from a coastal climate.