Transmission of Phytophthora ramorum in coast live oak woodlands
Jennifer M. Davidson1, Allison C. Wickland2, Andrea C. Morse2, Steven A. Tjosvold3, David L. Chambers3, Camille E. Jensen2, Garey Slaughter2, Matteo Garbelotto4, and David M. Rizzo2
Controlling the spread of Phytophthora ramorum requires understanding conditions for spore production and movement. For the past two years (2001-2002), we have sampled throughout the seasons for the presence of P. ramorum in rainwater, soil, and leaf litter at a study site in a coast live oak woodland in Sonoma Co., California. Number of colony forming units per liter rainwater were assessed approximately three times per month during the rainy season, and for each summer rain, by collecting rainwater in traps placed at twelve infected oak trees surrounded by infected bay trees. In both 2001 and 2002, there was a lag of several months between the start of the rains and the presence of detectable inoculum in rain samples. In addition, the greatest number of spores per liter rainwater occurred at the end of the rainy season. Build-up of infection on bay leaves as well as rising temperatures may contribute to the increase in spore production at the end of the rainy season. This phenomenon may be accentuated in El Niño years, during which rains occur late in spring, possibly resulting in high levels of spore production and corresponding waves of infection on oak trees. The predicted 2003 El Niño may provide an opportunity to test this hypothesis through continued rainwater sampling as well as a re-census of infected coast live oak trees on a one hectare plot at the same study site.
Soil and litter samples were collected monthly throughout the year at the same locale from around fifteen infected oak trees surrounded by infected bay trees. Soil was flooded with water and baited with pears to assess presence of P. ramorum. Positive soil and litter samples generally occurred in the same months as did positive rain samples. Samples spanning the dry season from June - December, 2001 were negative for P. ramorum. Results from the 2002 dry season are pending, but appear to follow the same pattern. Our corresponding laboratory experiments indicate that infested soil can have epidemiological significance: spores in soil spread to above ground leaves of bay seedlings via infection of green leaf litter. Knowledge of time periods in which P. ramorum spores are present and viable in rainwater or soil may help direct management decisions for containment.
1Pacific Southwest Research Station, USDA Forest Service, P.O. Box 245, Berkeley, CA 94701; jmdavidson@ucdavis.edu
2Department of Plant Pathology, One Shields Ave., University of California, Davis, CA 95616
3University of California Cooperative Extension, 1432 Freedom Blvd, Watsonville, CA 95076
4Department of Environmental Science, Policy and Management, Ecosystem Science Division, 151 Hilgard Hall, University of California, Berkeley, CA 94720
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