Sudden oak death, caused by the water mold Phytophthora ramorum, is a plant disease responsible for the death of hundreds of thousands of oak and tanoak trees. Some foliar hosts play a major role in the epidemiology of this disease. Upon infection by P. ramorum, these foliar hosts express non-fatal leaf lesions from which large amounts of inoculum can be produced and spread to neighboring host individuals, including oak species. Umbellularia californica (California bay laurel) may be one of the most important foliar hosts of sudden oak death due its observed ability to produce inoculum and its high abundance in the woodlands of coastal California. While previous research on susceptibility to P. ramorum in U. californica has shown significant variability among trees, with more resistant individuals in northern areas of its range, little is known about the causes or extent of this variability. Here, we ask three research questions: (1) How does susceptibility vary among U. californica individuals and P. ramorum isolates? (2) Are U. californica phenotype and genotype related to susceptibility? (3) What factors influence disease expression in nature
