Sudden oak death, a new forest disease in California

摘要

A forest disease caused by a newly described pathogen, Phytophthora ramorum, is presently affecting a number of woody plant species in central and northern coastal California and southern Oregon, U.S.A. Among the most severely affected tree speciesare coast live oak, Quercus agrifolia, and tanoak, Lithocarpus densiflorus. In March 2000, we established plots in Marin County, CA, to assess symptom progression and the association of other organisms with diseased trees. Symptomatic trees exhibit 'bleeding' of viscous sap from apparently intact bark, typically within approximately 2 m of the soil. At least three species of bark and ambrosia beetles colonize bleeding oaks. These beetles colonized every bleeding tree that died during the first year of the study (N = 23) while the foliage was still green. By March 2001, numbers of symptomatic and dead trees increased for both coast live oak and tanoak. Symptomatic coast live oaks totaled 26 percent in March 2000 and 27 percent in March 2001. During thesame period, mortality increased from 6 percent to 10 percent. For tanoaks, 41 percent were symptomatic in 2000, rising to 49 percent in 2001. Mortality was 11 percent in 2000 and 13 percent in 2001. The values for coast live oak are broadly consistent with independently acquired infection and mortality estimates derived from an unbiased transect method.