USDA Forest Service

Pacific Southwest Research Station

Pacific Southwest
Research Station

800 Buchanan Street
Albany, CA 94710-0011
(510) 883-8830
United States Department of Agriculture Forest Service. USDA logo which links to the department's national site. Forest Service logo which links to the agency's national site.

Research Topics Tree Mortality

Sudden Oak Death Research - Phytophthora ramorum

The walnut twig beetle excavates under bark to inoculate a tree with the Geosmithia morbida fungus, resulting in Thousand Cankers Disease.
Canyon live oak, Russian Ridge Open Space Preserve, San Mateo County, California. (Ted Swiecki/Phytosphere Research)

Since the mid-1990s, Phytophthora ramorum, the exotic plant pathogen that causes sudden oak death, has killed millions of oak and tanoak trees. Phytophthora ramorum also causes a non-lethal leaf or twig blight on over 125 plant species, including popular ornamental plants, such as rhododendrons and camellias.

With many of these popular ornamental host plants being shipped nationally and internationally, there is concern over inadvertent pathogen transport and introduction. In the United States, European Union, Canada and more than 60 other countries, federal and state quarantines are in effect to prevent pathogen hitch-hiking on plant materials.

In December 2002, with support from the Forest Service headquarters in Washington, D.C., and funding from Congress, PSW's Sudden Oak Death/Phytophthora ramorum research program began to address the emerging threat of sudden oak death. The research program aggressively investigated key issues associated with this quarantine pathogen, providing findings to support decision-making by land managers, arborists, homeowners, regulators, nurserymen, policy makers, and others.

Because this quarantine pathogen was new to science, most research was initially conducted in California, where the disease occurred in the natural environment. The PSW adopted a peer-reviewed, grant-based approach to provide a nimble mechanism by which the program direction could evolve as researchers answered early questions and new ones arose.

Goals and objectives

PSW's research program aimed at serving the needs of stakeholders impacted by P. ramorum, and organizations and individuals seeking to protect natural, economic, and social resources by prevention of the pathogen's introduction or establishment. Additionally, the program aimed to prevent the initiation of new damaging biological invasions by elucidating the lessons learned from this epidemic and by evaluating the risks of Phytophthoras to U.S. forests, communities, and industries.

The objectives were:

  1. Develop methods and strategies to prevent, treat, manage, and mitigate the impacts of P. ramorum, and respond to the hazards it creates;
  2. Improve early detection and monitoring methods for SOD and related diseases; and
  3. Elucidate the ecological, economic, and social impacts of P. ramorum and other forest Phytophthora species.


Sudden oak death science symposia

The Proceedings of the Sudden Oak Death Fifth Science Symposium has been published in two volumes:

  • Frankel, S.J.; Kliejunas, J.T.; Palmieri, K.M.; Alexander, J.M. tech. coords. 2013. Proceedings of the Sudden Oak Death Fifth Science Symposium. Gen. Tech. Rep. PSW-GTR-243. Albany, CA: U.S. Department of Agriculture, Forest Service, Pacific Southwest Research Station. 169 p.
  • Frankel, S.F. (ed). 2013. Tanoak: History, Ecology and Values. Madroño 60(2): 62-164. Articles found on pages 63, 87-94, 107-117, 130-138, 139-50, and 151-164 are available in Treesearch. *Due to policy or copyright restrictions, we are unable to provide a full-text version of this publication. Please check with your local library or contact Susan Frankel for information on obtaining this publication.

Prior symposia proceedings:

Sudden oak death references
A reference manual for managing sudden oak death in California (2013)

This publication contains background information and guidance for resource management professionals and landowners to understand and manage sudden oak death (SOD) in California forests. The publication is divided into three chapters: Chapter 1 discusses the epidemiology of SOD in California and includes information on biology of the pathogen Phytophthora ramorum, host-pathogen interactions, disease spread, and environmental conditions that affect disease development. An understanding of these relationships is needed to choose the most appropriate strategies for managing SOD at a given location. Chapter 2 describes how to develop a plan to manage SOD within a stand and how to identify and prioritize areas that may be suitable for SOD management activities. Options for managing SOD are presented by stage in the disease epidemic: before the SOD pathogen has reached a susceptible forest; during the local epidemic, while disease is active in an area and many hosts are still at risk of becoming diseased; and after SOD has killed so many host trees that forest restoration needs to be considered. Chapter 3 provides descriptions of management techniques.

Sudden oak death and Phytophthora ramorum: a summary of the literature (2010)

Sudden oak death and Phytophthora ramorum, both first recognized about a decade ago, have been the subject of hundreds of scientific and popular press articles. This document presents a comprehensive, concise summary of sudden oak death and P. ramorum research findings and management activities. Topics covered include introduction and background, identification and distribution, the disease cycle, epidemiology and modeling, management and control, and economic and environmental impacts.

Research needs assessment
2010 Research Needs Assessment for Sudden Oak Death/Phytophthora ramorum in Wildland and Nursery Environments (2010)

A decade has passed since the plant pathogen Phytophthora ramorum was recognized as the cause of Sudden Oak Death in the United States. In that time, Sudden Oak Death has killed more than an estimated 1 million trees in coastal California and Oregon (Meentemeyer 2008), and the pathogen has been detected in ornamental nurseries over 400 times. Despite quarantines established in 2002, P. ramorum remains a threat to forests and nurseries nationwide. The USDA-Forest Service, Pacific Southwest Research Station (PSW) has funded a P. ramorum research program since 2000 to provide a scientific basis for response programs and to develop management techniques to prevent or mitigate its effects.

  • Alexander, Janice; Lee, Chris; Suslow, Karen; Frankel, Susan J. 2010. 2010 Research Needs Assessment for Sudden Oak Death/Phytophthora ramorum in Wildland and Nursery Environments. Albany, CA: U.S. Department of Agriculture, Forest Service, Pacific Southwest Research Station. 6 p.
Evolution of an invasive species research program and implications for large-scale management of a non-native, invasive plant pathogen (2012)

We conducted a research needs assessment (RNA) in 2010 to gather opinions of "experts" and a larger public on research priorities for Phytophthora ramorum, the pathogen that causes sudden oak death in forest trees and Ramorum blight in ornamental plants. We place these 2010 findings in context with findings of similar P. ramorum needs assessments from 2002 and 2007-2008 and with a comprehensive literature review published in 2010. P. ramorum research needs have evolved from an emphasis on basic biological information toward an emphasis on management. As with many other non-native, invasive organisms, a major challenge remains how to move P. ramorum research into more wide-scale, unified attempts at management. Our analysis suggests that successfully moving from basic research to on-the-ground management requires overcoming the tendency toward specialized, limited viewpoints and providing stakeholders a comprehensive, integrated picture of the necessity and possibility of managing this plant disease.