A Survey of Phytophthora spp. Associated with Abies in U.S. Christmas Tree Farms

Multiple Phytophthora spp. are known to cause Phytophthora root rot (PRR) on Abies spp. Varying host susceptibilities and regional differences in Phytophthora community compositions complicate disease avoidance and management for Christmas tree growers and conifer nurseries. A survey of 32 Christmas tree farms in seven U.S. states representing four major Christmas tree production regions was conducted in 2012 to 2013 to characterize current distributions of root-rotting Phytophthora spp. and assess host responses to disease. Symptomatic tissues from lesion margins on roots and crowns were plated on selective medium to recover Phytophthora Cultures were identified by DNA sequencing of the nuclear internal transcribed spacer and mitochondrial cox I regions. Phytophthora cambivora was the prominent PRR species in western Washington and Oregon, while P. cinnamomi was prevalent in California and North Carolina. In the eastern United States, the undescribed P. taxon kelmania was isolated frequently and was capable of killing fir species considered tolerant of PRR. Completion of Koch's postulates confirmed pathogenicity of 12 previously unpublished Phytophthora-fir associations that were observed during this survey. Information regarding regional pathogen species prevalence and relative host susceptibilities will facilitate disease prevention by improving site management strategies for Phytophthora suppression.