Phytophthora Species Causing Tanoak Stem Cankers in Southwestern Oregon

Tanoak (Lithocarpus densiflorus) is a principal host of Phytophthora ramorum, cause of sudden oak death (SOD), in the western United States (1). In the course of SOD surveys in southwestern Oregon, other Phytophthora species were encountered to be causing stem cankers on tanoak that were indistinguishable from those caused by P. ramorum. In Oregon, SOD is subject to quarantine and eradication. Aerial surveys are flown two or more times a year to locate symptomatic tanoaks, which are then examined from the ground to determine the cause of death. Isolations on selective media were attempted from all trees with stem cankers typical of Phytophthora. Phytophthora species were identified by morphological features and DNA sequencing of either internal transcribed spacer (ITS) or the mitochondrial COX spacer region. ITS sequences were compared with validated GenBank records, and COX spacer sequences were compared with known reference isolates in the OSU collection. From 2001 through 2006, Phytophthora spp. were isolated from 482 of 1,057 tanoak stem cankers sampled. P. ramorum was isolated from 359 cankers, P. nemorosa was isolated from 102 cankers, P. gonapodyides was isolated from six cankers, P. cambivora was isolated from four cankers (all A1 mating type), P. siskiyouensis was isolated from four cankers, P. pseudosyringae was isolated from two cankers, P. cinnamomi was isolated from one canker (mating type A2), and P. taxon "Pgchlamydo" was isolated from one canker. Three cankers yielded isolates that were not identified but were closely related to P. pseudosyringae based on ITS sequence. No Phytophthora spp. were cultured from the remaining cankers. One isolate from each species identified (except P. ramorum and P. pseudosyringae) was tested for pathogenicity on tanoak stems (11.4 to 16.0 cm DBH) in the field. A 5-mm-diameter plug from the margin of a V8 agar culture was placed in a hole in the bark, covered with wet cheesecloth, and sealed with aluminum foil and duct tape. Each isolate was inoculated into five different stems. Each stem received three different isolates and an agar control. After 4 weeks, bark was removed to reveal lesion development. Lesions were measured (length by width), and pieces from four points on the lesion margin were plated in selective media to reisolate. P. cambivora, P. cinnamomi, P. gonapodyides, P. nemorosa, P. siskiyouensis and P. taxon "Pgchlamydo" all caused substantial lesions in inoculated tanoak trees (average area 11.5 to 18.6 cm²). In all cases, the species used for inoculation was recovered on reisolation from lesion margins. Control inoculations caused necrotic areas averaging 0.2 cm². Isolations from these areas were clean. Prior to the recent SOD epidemic, no species of Phytophthora were known as pathogens of tanoak. The discovery of P. ramorum as a pathogen of tanoak in California was quickly followed by the discovery that P. nemorosa and P. pseudosyringae were also associated with tanoak cankers (2). Six years of diagnostic support for survey and detection of P. ramorum in tanoak forests of southwest Oregon has revealed the occurrence, at very low frequency, of at least five additional species of Phytophthora causing stem cankers in tanoak. References: (1) D. M. Rizzo et al. Ann. Rev. Phytopathol. 43:309, 2005. (2) A. C. Wickland et al. For. Pathol. Online publication. DOI:10.1111/j.1439-0329.2008.00552.x), 2008.