Sudden Oak Death — Abstract of Paper


Sudden Oak Death: Disease Trends in Marin County Plots After One Year
Brice A McPherson,¹ David L. Wood,² Andrew J. Storer,³ N. Maggi Kelly,⁴ and Richard B. Standiford¹


Sudden oak death has emerged as a major threat to the oak forests of California. In oaks and tanoak, this disease complex consists of a previously unreported fungus-like pathogen, Phytophthora ramorum, insects (bark and ambrosia beetles), and a secondary fungus, Hypoxylon thouarsianum. Species monitored in this study were coast live oak (Quercus agrifolia), California black oak (Q. kelloggii), and tanoak (Lithocarpus densiflorus). Disease progression plots were initiated in March 2000 to determine infection and mortality levels, symptomology, and changes in disease status. Plots were placed in two ecologically different sites in Marin County, China Camp State Park and a protected watershed of the Marin Municipal Water District (MMWD). Ten plots of 0.1 to 0.2 ha, selected to encompass the range of habitat types and species compositions found within these forests, were placed in each site. For all oaks and tanoaks, the following data were recorded quarterly for one year: diameter (dbh); presence/absence and abundance of seeps; presence/absence of H. thouarsianum fruiting bodies; presence/absence of bark and ambrosia beetles; and condition of the foliage. More than 750 oaks and tanoaks in these plots have been permanently tagged and geolocated using GPS. Across China Camp State Park, independent and unbiased estimates of infection and mortality levels were acquired in summer 2001 using point-centered quarter sampling.

Apparent infection and mortality levels increased for the three species between March 2000 and March 2001. For coast live oak in China Camp, apparent infection levels increased from 35 percent in 2000 to 38 percent in 2001. In MMWD, these values were 16 percent in 2000 and 19 percent in 2001. Mortality of coast live oak rose from 8 to 15 percent in China Camp and 6 to 8 percent in MMWD during this period. California black oak in both sites exhibited apparent infection levels of 19 percent in 2000 and 27 percent in 2001. Apparent infection levels in tanoak rose from 40 to 55 percent in one year. Mortality also rose, from 12 to 15 percent. The point-centered quarter method yielded estimates of 30 percent for infection and 14 percent for mortality, for coast live oak. For California black oak, estimates were 21 percent for infection and 16 percent for mortality. The role of bark and ambrosia beetles in this disease complex is unclear. Every symptomatic tree that died had been colonized by beetles in the plots prior to death. These insects have consistently been found in association with seeping areas on infected trees. Normally these bark and ambrosia beetles are associated with dying, dead, and broken trees. These interactions between an apparently novel pathogen, its host plants, and native insects represent new ecological associations, with unknown future consequences for these forests.