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Sources of Inoculum for Phytophthora ramorum in a Redwood Forest

August 2008 , Volume 98 , Number  8
Pages  860 - 866

J. M. Davidson, H. A. Patterson, and D. M. Rizzo

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

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Accepted for publication 24 March 2008.

Sources of inoculum were investigated for dominant hosts of Phytophthora ramorum in a redwood forest. Infected trunks, twigs, and/or leaves of bay laurel (Umbellularia californica), tanoak (Lithocarpus densiflorus), and redwood (Sequoia sempervirens) were tested in the laboratory for sporangia production. Sporangia occurred on all plant tissues with the highest percentage on bay laurel leaves and tanoak twigs. To further compare these two species, field measurements of inoculum production and infection were conducted during the rainy seasons of 2003-04 and 2004-05. Inoculum levels in throughfall rainwater and from individual infections were significantly higher for bay laurel as opposed to tanoak for both seasons. Both measurements of inoculum production from bay laurel were significantly greater during 2004-05 when rainfall extended longer into the spring, while inoculum quantities for tanoak were not significantly different between the 2 years. Tanoak twigs were more likely to be infected than bay laurel leaves in 2003-04, and equally likely to be infected in 2004-05. These results indicate that the majority of P. ramorum inoculum in redwood forest is produced from infections on bay laurel leaves. Years with extended rains pose an elevated risk for tanoak because inoculum levels are higher and infectious periods continue into late spring.

Additional keywords:plant--host interaction, generalist pathogen.

© 2008 The American Phytopathological Society