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Forest Type Influences Transmission of Phytophthora ramorum in California Oak Woodlands

April 2011 , Volume 101 , Number  4
Pages  492 - 501

Jennifer M. Davidson, Heather A. Patterson, Allison C. Wickland, Elizabeth J. Fichtner, and David M. Rizzo

First author: Pacific Southwest Research Station, United States Department of Agriculture Forest Service, P.O. Box 245, Berkeley, CA 94701; and second, third, fourth, and fifth authors: Department of Plant Pathology, One Shields Ave., University of California, Davis 95616.

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Accepted for publication 20 October 2010.

The transmission ecology of Phytophthora ramorum from bay laurel (Umbellularia californica) leaves was compared between mixed-evergreen and redwood forest types throughout winter and summer disease cycles in central, coastal California. In a preliminary multisite study, we found that abscission rates of infected leaves were higher at mixed-evergreen sites. In addition, final infection counts were slightly higher at mixed-evergreen sites or not significantly different than at redwood sites, in part due to competition from other foliar pathogens at redwood sites. In a subsequent, detailed study of paired sites where P. ramorum was the main foliar pathogen, summer survival of P. ramorum in bay laurel leaves was lower in mixed-evergreen forest due to lower recovery from infected attached leaves and higher abscission rates of infected leaves. Onset of inoculum production and new infections of bay laurel leaves occurred later in mixed-evergreen forest. Mean inoculum levels in rainwater and final infection counts on leaves were higher in redwood forest. Based on these two studies, lower summer survival of reservoir inoculum in bay laurel leaves in mixed-evergreen forest may result in delayed onset of both inoculum production and new infections, leading to slower disease progress in the early rainy season compared with redwood forest. Although final infection counts also will depend on other foliar pathogens and disease history, in sites where P. ramorum is the main foliar pathogen, these transmission patterns suggest higher rates of disease spread in redwood forests during rainy seasons of short or average length.

Additional keywords: geographic mosaic, host × pathogen interaction, Lithocarpus densiflorus, microclimate, Quercus agrifolia, sudden oak death.

This article is in the public domain and not copyrightable. It may be freely reprinted with customary crediting of the source. The American Phytopathological Society, 2011.