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Sudden Oak Death — Abstract of Paper
Phytophthora ramorum and Sudden Oak Death in California: I. Host Relationships
David M. Rizzo,1 Matteo Garbelotto,2 Jennifer M. Davidson,1 Garey W. Slaughter,1 and Steven T. Koike3
A new canker disease of Lithocarpus densiflorus, Quercus agrifolia, Q. kellogii, and Q. parvula var. shrevei in California is shown to be caused by Phytophthora ramorum. The pathogen is a recently described species that was previously known only from Germany and The Netherlands on Rhododendron and Viburnum. This disease has reached epidemic proportions in mixed evergreen and redwood forests over an area approximately 300 km long along the central coast of California. The most consistent and diagnostic symptoms on larger trees are the cankers that develop before foliage symptoms become evident. Cankers have brown or black discolored bark, seep dark red sap and occur on the trunk at the root crown up to 20 m above the ground. Cankers do not enlarge below the soil line into the roots. Cankers can be over 2 m in length and are delimited by thin black zone lines in the inner bark. Foliage on affected trees often turns from a healthy green color to brown over a period of several weeks. In L. densiflorus saplings, P. ramorum was isolated from branches as small as 5 mm diameter. Lithocarpus densiflorus and Q. agrifolia inoculated with P. ramorum in the field and greenhouse developed symptoms similar to those of natural infections. The pathogen was re-isolated from inoculated plants, thereby confirming pathogenicity. Based on field observations and greenhouse inoculations, the host range of P. ramorum in California has been expanded and now includes Rhododendron spp., madrone (Arbutus menziesii), huckleberry (Vaccinium ovatum), manzanita (Arctostaphylos spp.), California bay laurel (Umbellularia californica), buckeye (Aesculus californica), bigleaf maple (Acer macrophyllum), toyon (Heteromeles arbutifolia), California coffeeberry (Rhamnus californica), and California honeysuckle (Lonicera hispidula). On these hosts, P. ramorum causes a variety of foliar and branch symptoms.
1Associate Professor, Post-doctoral Associate, and Staff Research Associate, respectively, Department of Plant Pathology, One Shields Ave., University of California, Davis, CA 95616 (email: dmrizzo@ucdavis.edu)
2Cooperative Extension Specialist, Department of Environmental Science, Policy and Management, Ecosystem Science Division, 151 Hilgard Hall, University of California, Berkeley, CA 94720.
3Farm Advisor, University of California Cooperative Extension, 1432 Abbott Street, Salinas, CA 93901.
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