Sudden Oak Death in California

Abstract

Phytophthora ramorum was first observed in 1993 as an undescribed species that caused shoot blighting of rhododendron in European nurseries. A genetically distinct lineage of P. ramorum was introduced into northern California nurseries in the same period. By 1995, P. ramorum had moved from infected nursery stock in the wildland interface into adjacent native coastal forests, where, it causes sudden oak death (SOD), a lethal trunk canker disease of several important hardwood trees in the Fagaceae. SOD has since become the single most important cause of mortality in these species. Tanoak (Notholithocarpus densiflorus) is the most severely impacted host because the entire P. ramorum disease cycle can be completed on this species and all age classes are susceptible. In contrast, susceptible oak (Quercus) species are infected and killed by inoculum produced on a separate foliar host, California bay (Umbellularia californica). Because SOD risk in oak stands depends on the density and distribution of California bay, disease risk can be greatly reduced or eliminated if bay canopy can be removed near oaks.