A Survey of Sudden Oak Death in Native California Forest and Woodland Communities: Relating Incidence and Intensity of Phytophthora ramorum to Plant Community and Site Variables
Shannon K. Murphy1 and David M. Rizzo2
Ten sites were surveyed for incidence of Phytophthora ramorum on all woody plant species in native forest and woodland plant communities. Sites were sampled in Alameda, Contra Costa, Marin, Mendocino, Napa, Santa Cruz, and Sonoma counties. Six of the sites were in California state parks, three were in county or regional parks, and one was on a private property protected with a conservation easement. Sampling occurred during the summer months of 2001 and 2002. At each site, twelve tenth hectare circular plots were surveyed. Plots were located along elevational gradients proceeding perpendicularly from river valleys toward ridge tops, with three plots along four transects at each site. Plots were selected based on aspect, plant community type, and areas with minimal human disturbance, rather than presence of P. ramorum. Each plot was evaluated for plant species composition, shrub and tree density, tree basal area, site variables, and presence of P. ramorum. Within each tenth hectare plot, a nested hundredth hectare circular plot was evaluated for tree density by plant species, soil/litter depth, and intensively sampled for percent incidence of P. ramorum in each woody plant species. Symptomatic leaf, stem, and bark tissue were assessed for presence of P. ramorum by culturing onto PARP media. Additional samples, as well as those with negative isolation results, were examined using PCR molecular detection protocols. Preliminary isolation results confirm P. ramorum presence at four of ten sites and within 14 of 120 plots. Symptomatic oaks with bleeding cankers characteristic of P. ramorum were observed at five additional sites but were not successfully isolated. P. ramorum was positively isolated from the following host species: Corylus cornuta, Heteromeles arbutifolia, Lithocarpus densiflorus, Quercus agrifolia, Quercus chrysolepis, Quercus parvula, Umbellularia californica, and Vaccinium ovatum. PCR detection will be completed in the fall and winter of 2002. Results from this study may provide additional information about the distribution of P. ramorum, including examination of location and intensity of P. ramorum within state and regional parks, and within east bay counties like Alameda and Contra Costa where Sudden Oak Death (SOD) plot studies have not previously been established. Additionally, this project may reveal new host species of P. ramorum, as well as provide information about whether or not infection in other host species can precede infection in Quercus spp. and Lithocarpus densiflorus. Furthermore, this study may identify influential plant community characteristics, as well as both biotic and abiotic site variables that contribute to the presence, spread, and intensity of the SOD disease. Information garnered in this study will provide additional basic information for ecological understanding and management of SOD.
1University of California, Department of Plant Pathology, One Shields Ave, Davis, CA 95616; (530) 754-9894; skmurphy@ucdavis.edu
2University of California, Department of Plant Pathology, One Shields Ave, Davis, CA 95616
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