First record of Phytophthora siskiyouensis associated with red alder ( Alnus rubra ) decline in British Columbia's coastal forests, Canada

Red alder (Alnus rubra Bong.) is the most abundant broadleaf tree species along the Pacific Northwest (PNW), including coastal British Columbia (BC), Canada, where it plays an important role as a pioneer species providing a wide range of benefits to riparian ecosystems. Its symbiotic relationship with nitrogen-fixing actinomycetes (Frankia sp.) contributes to soil fertility and ecosystem regeneration. Alder dieback has been attributed to multiple interacting biotic and abiotic agents, including oomycetes of the genus Phytophthora, which have been acting as predisposing or contributing factors in declining stands in the PNW (Sims et al. 2015; Feau et al. 2022) and Europe (Husson et al. 2015). In June 2024, a survey was conducted in a 0.7 ha stand of declining red alder trees in Daajing Giids (53.2569°N, 132.0919°W) on the Haida Gwaii archipelago in costal BC. Three Phytophthora-like cultures were obtained by plating root lesions on PARP(H)-CMA selective media (Ivors 2015) and by baiting soil samples collected from beneath declining red alder trees with pears. On V8 agar, colonies displayed a characteristic chrysanthemum-like growth pattern. Sporangia were predominantly ovoid, occasionally reniform, averaging 30.9 ±19.4 × 26.2 ±16.7 µm (n = 198), with a size range of 1.0-68.2 × 1.6-65.5 µm. Cultures were identified by DNA barcoding after growth in clarified V8-PARP(H) liquid medium. Genomic DNA was extracted from freeze-dried mycelia using DNeasy® Plant Mini Kit (Qiagen, Toronto, Ontario). The internal transcribed spacer (ITS) of the rDNA, elongation factor 1 alpha (EF1α) and beta tubulin (Btub) were amplified in three separate PCR reactions. PCR amplicons were sequenced in both directions using Sanger sequencing at the CHUL sequencing platform in Quebec City. BLASTn analysis of all three loci showed 99.82-100% identity with the P. siskiyouensis Reeser & E.M. Hansen ex-type isolates CPHST BL 56 (ITS) and 41B7 (EF1α and Btub). Sequences were deposited in GenBank under accession numbers PX875328-PX875330 (ITS), PX903677-PX903679 (EF1α), and PX883747-PX883749 (Btub). To verify Koch’s postulates, six one-year old A. rubra seedlings were inoculated with a single P. siskiyouensis isolate by inserting a mycelial plug into a small stem wound 10 cm above the soil line Seedlings were maintained at 20 °C in a growth chamber. Black, sunken lesions became visible externally three weeks after inoculation, and internal examination revealed phloem necrosis extending up to 4 cm above and below the inoculation point. The pathogen was successfully re-isolated from all inoculated A. rubra seedlings. Cultures and infected stems were deposited at the Pacific Forestry Centre’s Herbarium under accession numbers DAVP29865 to DAVP29867. No lesions developed on the three control seedlings inoculated with sterile V8-PARP(H) plugs. To our best knowledge, this is the first report of P. siskiyouensis associated with declining red alder in BC. This pathogen was first described from myrtlewood and tanoak in Oregon (Reeser et al. 2007) and has since been reported causing disease on Alnus spp. in Oregon and California (Rooney-Latham et al. 2009; Sims et al. 2015) and on grey alder in the UK (Perez-Sierra et al. 2015). This finding extends the known geographic distribution and host range of P. siskiyouensis in BC and highlights its potential as an emerging threat to red alder ecosystems, with possible implications for riparian stability and alder-associated ecological functions.