Biomarkers identify coast live oaks that are resistant to the invasive pathogen Phytophthora ramorum

California coast live oaks (Quercus agrifolia) trees have suffered large losses from sudden oak death, caused by the introduced oomycete Phytophthora ramorum. In this review paper, we discuss oak plant chemistry as a potential predictor of disease susceptibility. We have recorded an annual mortality rate of three percent in long-term monitoring plots in Marin County, resulting in greater than 40 percent loss since 2000. Despite this mortality rate, asymptomatic trees still persist in many heavily infected stands. We hypothesized that varying responses to P. ramorum, including apparent recovery from infections, reflected phenotypic differences in susceptibility. In a Marin County inoculation study, a logit model showed that external canker lengths measured 9 months following inoculation predicted both resistance and survival 7 years later. The distribution of canker length was consistent with quantitative resistance to P. ramorum. The role of plant chemistry in resistance was examined by quantifying soluble phenolics in phloem methanol extracts prepared from the surviving trees. A logistic regression model found that expression of resistance was associated with total phenolics and four phenolic compounds; ellagic acid, a partially characterized ellagic acid derivative, and two chromatographic peaks representing two uncharacterized phenolic compounds. In vitro tests showed that ellagic acid was fungistatic against P. ramorum and total phenolics were fungicidal at physiologically relevant concentrations. A subsequent inoculation study in Briones Regional Park, Contra Costa County, California, showed that some of the same compounds were correlated with resistance. The association of certain phenolics with resistance may facilitate the use of biomarkers in minimally invasive assays to predict the response of trees to P. ramorum, thereby increasing the options for managing threatened forests.