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    Author(s): Ebba K. Peterson
    Date: 2011
    Source: Doctor of Philosophy Dissertation. Oregon State University, Corvallis, OR. December 5, 2011. 152 p.
    Publication Series: Other
    Station: Pacific Southwest Research Station
    PDF: Download Publication  (2.18 MB)


    The phytopathogen Phytophthora ramorum (Werres, DeCock & Man in't Veld), causal agent of Sudden Oak Death (SOD) of oaks (Quercus spp.) and tanoaks (Notholithocarpus densiflorus syn. Lithocarpus densiflorus), is established in coastal forests of the western United States. Since the discovery of SOD in the Douglas-fir / tanoak forests of southwest Oregon in 2001, a multiagency effort has ensued with the goal of fully eliminating P. ramorum from this originally small and isolated area. In this study we investigated the epidemiology of SOD in Oregon, particularly as it affects the success of the eradication program. Two approaches were taken to discern the mechanism of long distance dispersal: first, a landscape analysis of the spatial relationship between SOD sites and roads or streams, features associated with movement of infested soils, and, second, a local analysis to discern if understory infection is originating from soil or stream-borne inoculum. Using a restricted randomization test we concluded that SOD sites were no closer to roads than expected by chance, which is inconsistent with soil dispersal by people. While we found evidence that SOD sites were preferentially closer to waterways, inoculum had not moved away from streams into adjacent understory foliage. The local distribution of understory infection around SOD positive trees indicated that primary inoculum is infecting overstory canopies first, suggesting that P. ramorum is dispersing in air currents. Regression modeling indicated that weather conditions two years before detection could explain variation in the maximum distance inoculum moved each year of the epidemic between 2001 and 2010. This two year delay between infection and detection has allowed ample time for infested sites to contribute to further spread. Model results were consistent with observations made the summer of 2011, when trees likely infected by secondary inoculum at non-eradicated sites developed symptoms but were still undetectable by aerial surveys. Due to the prevalence of infection on tanoak, opportunities for sporulation and infection occur more often in Oregon than in California. These data can explain the failure to eliminate P. ramorum. Nevertheless, we did find evidence that the eradication program has significantly reduced the potential size of the SOD epidemic in Oregon. [research funding through FS Joint Venture Agreement 10-JV-066 ]

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    Peterson, Ebba K. 2011. The epidemiology of sudden oak death in Oregon forests. Doctor of Philosophy Dissertation. Oregon State University, Corvallis, OR. December 5, 2011. 152 p.


    sudden oak death, tanoak, forest Phytophthora, P. ramorum, Oregon

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