During August 2002, Phytophthora ramorum S. Werres & A.W.A.M. de Cock was isolated from branches <2.0 cm in diameter on a canyon live oak (Quercus chrysolepis) in Mt. Tamalpais State Park, Marin County, CA. The shrub was a cluster of stems <1 m in diameter and 1 m high. Similar cankers were observed on small branches of adjacent canyon live oaks and there was dieback of the branches distal to the lesions. Many tanoak (Lithocarpus densiflorus), California bay laurel (Umbellularia californica), and evergreen huckleberry (Vaccinium ovatum) were also infected by P. ramorum at this site. The isolate was identified as P. ramorum by its abundant chlamydospores and caducous, semi-papillate sporangia and internal transcribed spacer (ITS) rDNA sequences identical to those of isolates of P. ramorum from Quercus spp., tanoak, and Rhododendron (1,3). To test for pathogenicity, two greenhouse trials (5 seedlings per trial plus controls) were conducted on 20- to 24-month-old canyon live oak seedlings. Coast live oak (Q. agrifolia, section Lobatae) seedlings were included in the trials as a comparison because the species is known to be susceptible (1). Stems (approximately 1 cm in diameter) were wound inoculated (1). After 6 weeks, lesion lengths in the cambium of canyon live oak averaged 17.2 mm (range 16 to 30 mm), which was significantly greater (analysis of variance [ANOVA], P < 0.05) in both trials than those of control inoculations (mean = 6 mm). Coast live oak seedlings inoculated at the same time had mean lesion lengths of 22.6 mm (range 15 to 30 mm). P. ramorum was recovered from 100% of inoculated stems. Canyon live oak has a wide geographic range within California, but is not common in the areas currently affected by P. ramorum. We have not observed disease symptoms or unusual mortality on overstory canyon live oaks. Although a number of understory canyon live oaks at the site on Mt. Tamalpais were apparently infected, the long-term effect of P. ramorum infection on understory trees remains unclear. To our knowledge, this is the first report of infection by P. ramorum of an oak species outside of the section Lobatae (red oaks); canyon live oak is classified in the section Protobalanus (intermediate or golden cup oaks) (2). Oaks in the section Quercus (white oaks) have not been observed to be infected by P. ramorum in the field.
References: (1) D. M. Rizzo et al. Plant Dis. 86:205, 2002. (2) P. Manos et al. Mol. Phylogenet. Evol. 12:333, 1999. (3) S. Werres et al. Mycol. Res. 105:1155, 2001.
