Disease incidence and spatial distribution of host resistance in a coast live oak/sudden oak death pathosystem

Abstract

Sudden oak death (SOD) results in extensive mortality of native populations of red oak (Quercus spp.) and tanoak (Notholithocarpus densiflorus) in coastal California and Oregon. The pathogen, Phytophthora ramorum, causes a syndrome in Q. agrifolia (coast live oak, CLO) characterized by bleeding stem cankers, attacks by bark and ambrosia beetles, and development of the endophytic fungus, Annulohypoxylon thouarsianum. The study examined disease incidence and resistance in CLO within Northern California stands that had no apparent prior exposure to the pathogen. Seven years after artificial inoculation of mature trees distributed between two separate stands in a California wildland, 27% of CLO expressed resistance to P. ramorum, while 61% died (N = 149). The remaining trees were alive but symptomatic. External and subcortical canker lengths, measured approximately one year post-inoculation, were significant predictors of CLO resistance and survival seven years post-inoculation. Spatial analysis also revealed that variation in CLO susceptibility to P. ramorum is aggregated on the landscape, suggesting that more resistant and susceptible trees tend to co-occur and that resistance is a heritable trait. From 2011 to 2017 the incidence of natural infections in a second cohort of non-inoculated trees increased from 2.0% (N = 447) to 13.2% (N = 423). Altogether, these findings suggest that estimating the frequency and determining the spatial distribution of resistant trees on the landscape can be used to identify sites that should be targeted for germplasm collection and habitat conservation.