Increasing Distance from California Bay Laurel Reduces the Risk and Severity of Phytophthora ramorum Canker in Coast Live Oak

Foliar infections in California bay (Umbellularia californica) are the most important known source of inoculum contributing to Phytophthora ramorum canker in coast live oak (Quercus agrifolia). This research addressed the question whether there is a “safe” distance between California bay and coast live oak beyond which the risk of disease is acceptably low. We quantitatively evaluated bay cover and other factors in the neighborhoods around 247 coast live oaks in long term research plots in mixed hardwood forests where P. ramorum canker has been prevalent since 2000. Both the risk and severity of P. ramorum canker decreased as the minimum distance between California bay foliage and the oak trunk increased. Disease risk and severity were greatest at bay foliage-oak trunk distances of 1.5 m or less and were minimal at a distance of 10 m or more. Bay cover within 2.5 m of the trunk was a stronger predictor of disease risk and severity than the minimum bay-trunk distance. These results suggest that removing bay from within 2.5 m of the trunk of a susceptible oak will greatly reduce, but not eliminate, the risk of disease. For some oaks with P. ramorum canker, the presence of disease symptoms could not be readily explained by proximity to bay, but large amounts of poison oak (Toxicodendron diversilobum) vines climbing in the oak canopy or in adjacent trees appeared to be the most likely source of inoculum. Based on timed counts of symptomatic bay leaves repeated at intervals between fall 2005 and fall 2006, bay foliar infection levels were minimal in fall and peaked in late spring and summer. Counts of infected leaves in fall 2005 were not correlated with counts from the same trees in either spring/summer 2006 or fall 2006, but spring/summer 2006 counts were correlated with fall 2006 counts.