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Additional key words: Umbellularia californica, Quercus agrifolia, Toxicodendron diversilobum,disease risk, disease severity, cover, clearance, oak decline
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 of 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. Risk of tree mortality due to P. ramorum canker was highest for bay foliage-oak trunk distances less than 0.5 m.
Disease risk and disease severity increased as bay cover within 2.5 m of the trunk increased. Bay cover within 2.5 m of the trunk was a stronger predictor of disease risk and severity than the minimum bay-trunk distance. Bay foliage located south and west of an oak (prevailing wind directions during storms) had greater influence on disease incidence and severity than bay foliage that was north or east of the oak. The strength of this directional effect was greater for bay foliage 2.5-10 m from the oak trunk than for bay foliage within 2.5 m of the trunk.
For some oaks with P. ramorum canker, the presence of disease symptoms could not be readily explained by proximity to bay. 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 P. ramorum inoculum for these trees.
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 bay leaves in fall 2005 were not correlated with counts from the same trees in either spring/summer 2006 or fall 2006. Spring/summer 2006 counts were correlated with fall 2006 counts.
Based on our results, the risk of developing P. ramorum canker in coast live oak appears to be minimal at bay foliage-oak trunk clearances of 10 m or more. The risk of P. ramorum canker and the severity of the disease can be greatly reduced, but not completely eliminated by (1) removing bay from within 2.5 m of the trunk of a susceptible oak; (2) extending bay foliage-oak trunk clearance to 5 m where possible, especially in the direction(s) from which storm winds blow; (3) pruning low branches to obtain up to 5 m of clearance in the lower canopy even if upper canopy bay branches are present at closer horizontal distances; and (4) eliminating poison oak climbing at canopy level within an oak or in adjacent tree within 2.5 m of the oak trunk.