Progression of Sudden Oak Death Over Two Years at Sites in Marin County, California
Brice A. McPherson1, Richard B. Standiford2, David L. Wood3, and Andrew J. Storer4
Disease progression plots were established in two sites in Marin County, CA, in March 2000 to follow changes in symptoms of sudden oak death, caused by Phytophthora ramorum, in oaks and tanoaks. Twenty plots were equally divided between China Camp State Park, along San Pablo Bay, and Marin Municipal Water District (MMWD) land, near Mt. Tamalpais. These plots included 507 coast live oaks, Quercus agrifolia, 45 California black oaks, Q. kelloggii, and 166 tanoaks, Lithocarpus densiflorus, over 2 inches (5 cm) diameter. Each tree was evaluated every three months. Variables included bleeding status, presence of ambrosia and bark beetles, reproductive structures of the fungus Hypoxylon thouarsianum, and foliage condition. Bleeding cankers are the most reliable and consistent symptom of P. ramorum infection in Q. agrifolia and Q. kelloggii, and to a lesser extent, L. densiflorus. In both Q. agrifolia and L. densiflorus, the numbers of both symptomatic and dead trees increased during the monitoring period. Summed across all plots, symptomatic (bleeding) Q. agrifolia in the 10 China Camp plots accounted for 35 % (range 11 % to 95 %) in March 2000, and 34 % (range 15 % to 100 %) of living trees in March 2002. In the 8 MMWD plots, these values for Q. agrifolia were 14 % (range 0 % to 45 %) in 2000, and 18 % (range 0 % to 65 %) in 2002. For the 6 plots with L. densiflorus in MMWD, symptomatic trees constituted 46 % (range 20% to 64 %) in 2000 and 61 % (range 47% to 89%) in 2002. In both sites, combining symptomatic trees and those that died with bleeding symptoms, the total impact of the disease for Q. agrifolia was 31% (2000), rising to 36% (2002). Total disease impact for L. densiflorus was 48% (2000), rising to 67% (2002). Because the sample size for Q. kelloggii is small in this study, comparable values are less reliable, but the response of this species appears to be similar to that of Q. agrifolia.
Even though P. ramorum was already well established in these areas of Marin County before plots were initiated, considerable change is evident after 2 years. The number of symptomatic trees has continued to increase, as has the number of trees that have died with symptoms of P. ramorum infection. There is a consistent association of bark and ambrosia beetles with the bleeding cankers of infected Q. agrifolia. The significance of these insects in disease progression is unknown, but every symptomatic tree that died through June 2002 (n=37) had been colonized by these beetles while foliage was green. A number of trees that were bleeding and had been colonized by March 2000 were still producing apparently healthy foliage in June 2002, indicating that the duration of infected tree survival is variable, and can be much greater than 2 years. This conclusion is supported by the fact that no trees have progressed from asymptomatic to infected, and then to dead during the first two years of this study.
1Department of Environmental Science, Policy, and Management, IHRMP/Center for Forestry, 145 Mulford Hall, University of California, Berkeley, CA 94720-3114; (510) 642-5806; aoxomoxo@nature.berkeley.edu
2Department of Environmental Science, Policy, and Management, IHRMP/Center for Forestry, 145 Mulford Hall, University of California, Berkeley, CA 94720-3114
3Department of Environmental Science, Policy, and Management, Division of Insect Biology, University of California, Berkeley, CA 94720
4School of Forest Resources and Environmental Science, Michigan Technological University, 1400 Townsend Dr., Houghton, MI 49931
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