Source or sink? The role of soil and water borne inoculum in the dispersal of Phytophthora ramorum in Oregon tanoak forests

•Spread of inoculum along roads or streams threatens the containment of P. ramorum.•Understory infection must precede overstory mortality if inoculum moves in soils.•Absent overstory sources P. ramorum was not recovered, even in high risk areas.•Aerial detection of overstory tanoak describes the distribution of SOD in Oregon.•Disease gradients suggest dispersal of inoculum occurs in blowing wind or fog.