Impacts of Phytophthora ramorum Canker and Other Agents in Sonoma County Forests

To study impacts of sudden oak death (SOD), a lethal bark canker disease caused by Phytophthora ramorum, we established permanent plots in Sonoma County forest types at risk of SOD. Baseline stand and tree health data were collected in 2001 and the plots were reassessed in 2004. The 250 plots (0.02 ha each) were located at 11 study locations in stands containing Quercus agrifolia, Q. kelloggii, or Lithocarpus densiflorus as the dominant hardwood species. By 2004, P. ramorum canker symptoms developed at two locations that lacked symptoms in 2001, leading to new tree mortality at one of these locations. Between 2001 and 2004, plot level incidence of P. ramorum canker increased from 29 to 40 percent of plots containing L. densiflorus and from 2 to 10 percent in plots containing Q. kelloggii. Plots with Q. agrifolia showed a slight drop in P. ramorum canker (from 9 to 7 percent of plots) due to apparent symptom remission in trees at one location. Between 2001 and 2004, the percentage of trees with P. ramorum canker symptoms increased at three of four locations with symptomatic SOD canker hosts. Mortality due to both P. ramorum and other agents increased at 9 of 11 study locations between 2001 and 2004. Among SOD canker hosts that died during this period, mortality was due to P. ramorum in 4 of 16 Q. kelloggii, 7 of 18 Q. agrifolia, and 18 of 50 L. densiflorus. In most study locations, annual background mortality unrelated to P. ramorum was less than 1 percent per year between 2001 and 2004. Over this same period, mortality due to P. ramorum exceeded background mortality at three locations and equaled it at a fourth. At one L. densiflorus location, mortality associated with an unidentified bark canker was comparable to levels of mortality associated with P. ramorum at other locations. Only P. nemorosa and P. pseudosyringae were isolated at this location.