First Report of Armillaria cepistipes Causing Root Disease on Populus trichocarpa (Black Cottonwood) in Oregon, U.S.A.

Populus trichocarpa Torr. and Gray (black cottonwood) is an economically and ecologically important tree species native to western North America. It serves as a model tree species in biology and genetics due to its relatively small genome size, rapid growth, and early reproductive maturity (Jansson and Douglas 2007). Black cottonwood is susceptible to root rot caused by at least one species of Armillaria (Raabe 1962), a globally distributed genus that exhibits diverse ecological behaviors (Klopfenstein et al. 2017) and infects numerous woody plant species (Raabe 1962). However, several Armillaria spp. have been isolated from Populus spp. in North America (Mallet 1990), and the most recent report of Armillaria on P. trichocarpa used the now ambiguated name A. mellea (Vahl.) Quel. (see Raabe 1962). In April 2016, mycelial fans and rhizomorphs of an unknown Armillaria species (isolate WV-ARR-3) were collected from P. trichocarpa in a riparian hardwood stand ca. 5.5 km east of Springfield, Oregon, USA (44°3'21.133"N, 122°49'39.935"W). The host was dominant in the canopy, large in diameter (ca. 90-cm dbh) relative to neighboring trees, and exhibited minimal crown dieback (ca. < 5%). A mycelial fan was observed destroying living cambium beneath the inner bark, indicating pathogenicity. The isolate was cultured on malt extract medium (3% malt extract, 3% dextrose, 1% peptone, and 1.5 % agar) and identified as A.cepistipes on the basis of somatic pairing tests and translation elongation factor 1α (tef1) sequences (GenBank Accession No. MK172784). DNA extraction, PCR, and tef1 sequencing followed protocols of Elias-Roman et al. (2018). From nine replications of somatic incompatibility tests (18 tester isolates representing six North American Armillaria spp.), the isolate showed high intraspecific compatibility (colorless antagonism) with three A. cepistipes tester isolates (78%), but low compatibility with the other Armillaria spp. (0 - 33%) that occur in the region. Isolate WV-ARR-3 yielded tef1 sequences with a 99% identity to A. cepistipes (GenBank Accession Nos. JF313115 and JF313121). A second isolate (WV-ARR-1; GenBank Accession No. MK172783) with a nearly identical sequence was collected from a maturing P. trichocarpa in a riparian stand ca. 8 km northeast of Monroe, Oregon (44°21'47.57"N, 123°13'14.415"W) along the Willamette River, downstream from the McKenzie river tributary where WV-ARR-3 was collected. Armillaria cepistipes has been reported on Alnus rubra (red alder) in Washington, USA (Banik et al. 1996) and on broad-leaved trees in British Columbia, Canada (Allen et al. 1996). It is generally considered to be a weak pathogen on broad-leaved trees in the Pacific Northwest, but it is also associated with pathogenicity on both coniferous and deciduous trees in Europe (e.g., Lygis et al. 2005). However, a recent phylogenetic study suggested that North American A. cepistipes is phylogenetically distinct from Eurasian A. cepistipes (Klopfenstein et al. 2017), butadditional studies are needed to determine the formal taxonomic status of North American A. cepistipes. To our knowledge, A. cepistipes has not been previously confirmed on P. trichocarpa in the U.S.A. or formally reported as a pathogen of any Populus species in North America. Continued studies are needed to determine the distribution, host range, and ecological role of A. cepistipes in riparian forests of the Pacific Northwest, while monitoring its populations under changing climates.