Intraspecific variation in host susceptibility and climatic factors mediate epidemics of sudden oak death in western US forests

Umbellularia californica is one of the key infectious hosts of the exotic Phytophthora ramorum, which causes sudden oak death (SOD) in California and Oregon forests. This study provides a comprehensive analysis of the epidemiologically relevant parameters for SOD in California and southern Oregon, including potential differences between the two states. Experimental infection of U. californica leaves was optimal when leaves were wet for 6–12 h, temperature was approximately 19°C and pathogen concentration was approximately 2·7 × 10⁴ zoospores mL^?1. Seasonal variation in host susceptibility and disease incidence was examined for two populations by inoculating detached leaves at 12 dates and by monitoring naturally infected leaves, respectively. Susceptibility of U. californica and disease incidence varied significantly in time and the variation was highest for both in spring. Susceptibility of trees from 17 natural populations from California and southern Oregon was assessed in detached leaf inoculations. One California and three southern Oregon populations had significantly and repeatable lower average susceptibility in artificial inoculations, but differences among three selected California and Oregon populations were not significant in inoculations of seedlings grown from seed in a common garden. This study concludes that U. californica susceptibility has a large environmental component, yet still predicts potential disease severity in different sites especially where infestations are young or the pathogen has not yet arrived. The accuracy and utility of predictive risk models for P. ramorum will be enhanced by the inclusion of both the environmental and host susceptibility components.