Jennifer M. Davidson1, Patricia E. Maloney2, Allison C. Wickland2, Andrea C. Morse2, Camille E. Jensen2, and David M. Rizzo2
Quercus agrifolia (coast live oak) and Umbellularia californica (bay laurel) are dominant components of coast live oak woodlands in California and may comprise over 80% of the individuals (> 1 cm dbh) at a site. Both of these tree species are hosts for Phytophthora ramorum, the causal agent of Sudden Oak Death. However, experiments indicate that only infections on bay leaves may be important for production of spores and transmission of disease.
To better understand transmission of P. ramorum in leaves of bay, we marked 1000 asymptomatic bay leaves at the end of January 2002, in each of ten sites for a total of 10,000 leaves, to monitor infection and leaf fate (abscission) during the rainy season and subsequent dry season. Of the ten sites, four were coast live oak-bay forests, five were tanoak-redwood forests, and one site was a mixture of the two forest types. The bay leaves were censused in April and July 2002 for Phytophthora symptoms and abscission. (A final census will be performed in October 2002). A small piece (approximately 9 mm2) of symptomatic leaves was removed for verification of P. ramorum using selective medium (PARP).
During the period of January through April, 360/10,000 (3.60%) leaves became infected with P. ramorum. From April through July, another 114 (1.14%) were infected. Leaves that were infected by the first census date were 15 times more likely to be shed by the second census date than were non-infected leaves. Furthermore, an infected bay leaf in coast live oak forest was significantly more likely to be dropped than an infected bay leaf in tanoak-redwood forest. In two additional, separate experiments in a coast live oak-bay forest in Sonoma Co., we looked at survival of P. ramorum throughout the dry season in bay leaves that were attached or had abscised. Survival was assessed by plating of leaves on PARP. Survival in attached bay leaves declined, but persisted throughout the summer from approximately 90% in June to 50% in August. Survival in abscised leaves collected from leaf litter in June, July, and August was nearly zero. Given that survival of P. ramorum occurs in attached rather than abscised leaves, differences in abscission rates of P. ramorum infected bay leaves in coast live oak versus tanoak-redwood forests may be one factor contributing to differences in the onset of detectable inoculum production in these two forest types during the winter rainy season.
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
