In coastal California, infection by the pathogen Phytophthora ramorum, causal agent of sudden oak death, results in extensive mortality of native oak species including Quercus agrifolia (coast live oak). However, apparently resistant Q. agrifolia have been observed within native populations. In this study (Conrad and others 2019), we monitored disease progression from 2010 to 2017 in Q. agrifolia artificially inoculated with P. ramorum and disease incidence in Q. agrifolia left to become naturally infected in the same stand. After seven years, 61% of artificially inoculated Q. agrifolia died while 27% appeared to be resistant (i.e. in remission, no longer showing active symptoms of P. ramorum infection) (N = 149). In addition, 13% of noninoculated Q. agrifolia showed symptoms of natural P. ramorum infection, e.g. bleeding exudate (N = 423). Canker length measured approximately one year following inoculation was a significant predictor of Q. agrifolia resistance and survival (P < 0.001). Canker length was also used to examine the distribution of resistant and susceptible Q. agrifolia across the landscape using inverse distance weighted analysis. This analysis revealed resistant and susceptible Q. agrifolia are aggregated, suggesting resistance is a heritable trait. A better understanding of the amount and distribution of resistant Q. agrifolia within native populations can be used to facilitate the restoration of disturbed habitats and identify sources of germplasm for future breeding efforts.
Conrad, Anna O.; D’Amico, Katherine M.; Bonello, Pierluigi; McPherson, Brice A.; Wood, David L.; Lopez-Nicora, Horacio D. 2020. Incidence and distribution of resistance in a coast live oak/sudden oak death pathosystem. In: Frankel, Susan J.; Alexander, Janice M., tech. cords. Proceedings of the seventh sudden oak death science and management symposium: healthy plants in a world with Phytophthora. Gen. Tech. Rep. PSW-GTR-268. Albany, CA: U.S. Department of Agriculture, Forest Service, Pacific Southwest Research Station: 13.