Sudden oak death in California: Disease progression in oaks and tanoaks

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Sudden oak death (SOD), caused by Phytophthora ramorum, is killing oaks and tanoaks in the Coast Ranges of California, from Monterey County to Humboldt County. In March 2000, 20 disease progression plots were established in Marin County, California, to characterize the progress of disease symptoms, and to determine the fidelity of the association of three or more bark and ambrosia beetle species (Coleoptera: Scolytidae) with diseased oaks and tanoaks. Symptoms of sudden oak death and signs of associated organisms were recorded from coast live oaks (Quercus agrifolia), California black oaks (Q. kelloggii), valley oaks (Q. lobata), and tanoaks (Lithocarpus densiflorus), four times per year, from March 2000 through March 2003. Symptoms and signs in Q. agrifolia progressed from bleeding, to infestation by scolytid beetles, to the development of fruiting structures of the fungus Hypoxylon thouarsianum. Mortality of symptomatic trees increased from 2000 to 2003 as follows: Q. agrifolia (n = 668), 5.8-17.4%; Q. kelloggii (n = 53), 3.8-9.4%; and L. densiflorus (n = 164), 8.3-22.2%. All 31 Q. lobata remained asymptomatic. From 2000 to 2003, bleeding trees were 25.0-23.6% of living Q. agrifolia, 15.5-25.0% of Q. kelloggii, and 39.0-62.4% of L. densiflorus. Scolytid beetles colonized more than 95% of the living symptomatic Q. agrifolia that subsequently died. Same-symptom cohorts were followed from March 2000 through March 2003. In the asymptomatic Q. agrifolia cohort, 12.0% developed bleeding by 2003. For the bleeding only cohort, 22.7% of Q. agrifolia died, but 73.5% of the beetle-colonized bleeding cohort died. Bleeding developed in 40.9% of the initially asymptomatic L. densiflorus cohort. By 2003, 24.6% of the initially bleeding L. densiflorus cohort had died. Both Weibull and Cox Proportional Hazards regression were used to model cohort survival. The median survival time estimated by Weibull regression models declined rapidly by disease category (asymptomatic, bleeding only, bleeding with beetles), from 29 to 2.7 years for Q. agrifolia, and from 12.6 to 2.9 years for L. densiflorus. By 2003, structural bole failure had occurred in 21.5% of the Q. agrifolia that were bleeding in 2000, 93% of which had ambrosia beetle tunnels at the breakage point. For both Q. agrifolia and L. densiflorus, health failure analysis modeled by Weibull regression found a greater probability of developing sudden oak death for trees with larger stem diameters. Beetles were also positively correlated with larger diameter bleeding Q. agrifolia. © 2005 Elsevier B.V. All rights reserved.

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Forest Ecology and Management