Phytophthora species associated with red alder dieback in British Columbia, Canada

Red alder (Alnus rubra) trees form an essential component of the communities along coastal streams in the Pacific Northwest. However, canopy dieback has been observed in red alder for several years. Symptomatic trees found at two sites in the greater Vancouver area, British Columbia, Canada, were investigated for disease-causing organisms. We investigated the presence of causal agents by isolating potential pathogens using direct plating of root samples on Petri dishes and by baiting for oomycetes in soil and root samples, followed by DNA barcoding. We identified three pathogenic Phytophthora species: Phytophthora cambivora, P. syringae and P. pseudosyringae. Controlled inoculations of all three on red alder leaves indicated that only P. pseudosyringae has the potential to cause both stem canker and aerial symptoms. This is the first isolation of the tree pathogen P. pseudosyringae in Canada. Additionally, the inoculation experiments revealed that P. gonapodyides, a species that is frequently detected in riparian ecosystems with diseased alders in Oregon but not directly associated with tree mortality, was more aggressive than the three pathogenic Phytophthora species tested. Our results reveal a greater complexity than previously suspected in red alder decline.

Automated summary

Red alder trees are an important part of coastal stream communities in the Pacific Northwest. However, canopy dieback has been observed in red alder for several years. Through investigations of symptomatic trees in Vancouver, Canada, three pathogenic Phytophthora species were identified, with P. pseudosyringae being the only species capable of causing both stem canker and aerial symptoms. Additionally, a non-lethal species, P. gonapodyides, was found to be more aggressive than the pathogenic species tested. These results reveal a greater complexity in red alder decline than previously suspected.