Alveshere Brandon, Bennett Patrick, Kim Mee-Sook, Klopfenstein Ned B, LeBoldus Jared M
Oregon State University, 2694, Forest Engineering Resources and Management, 140 Peavy Hall, 3100 SW Jefferson Way, Corvallis, Oregon, United States, 97333;
Oregon State University, 2694, Botany and Plant Pathology , 2082 Cordley Hall, 2701 sw campus way, Oregon State university, Corvallis, Oregon, United States, 97331;
Plant Dis. 2021 Feb 22. doi: 10.1094/PDIS-09-20-1993-PDN.
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 Elías-Román 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.
毛果杨（Populus trichocarpa Torr. and Gray），又称黑杨，是原产于北美西部的一种在经济和生态方面都很重要的树种。由于其基因组相对较小、生长迅速且生殖成熟较早，它在生物学和遗传学领域作为模式树种（扬松和道格拉斯，2007年）。黑杨易受至少一种蜜环菌属（Armillaria）真菌引起的根腐病影响（拉贝，1962年），该属真菌分布于全球，具有多样的生态行为（克洛普芬斯坦等人，2017年），并能感染众多木本植物物种（拉贝，1962年）。然而，在北美已从杨树属（Populus spp.）中分离出几种蜜环菌（马利特，1990年），关于毛果杨上蜜环菌的最新报告使用了现已模糊不清的名称蜜环菌（Armillaria mellea (Vahl.) Quel.）（见拉贝，1962年）。2016年4月，在美国俄勒冈州斯普林菲尔德以东约5.5公里处（北纬44°3'21.133"，西经122°49'39.935"）的河岸硬木林中，从毛果杨上采集到一种未知蜜环菌（菌株WV - ARR - 3）的菌丝扇和菌索。该寄主在林冠层中占优势，相对于相邻树木直径较大（胸径约90厘米），且树冠枯萎程度极小（约<5%）。观察到一个菌丝扇正在破坏内皮下方的活形成层，表明具有致病性。将该菌株在麦芽提取物培养基（3%麦芽提取物、3%葡萄糖、1%蛋白胨和1.5%琼脂）上培养，并根据体细胞配对试验和翻译延伸因子1α（tef1）序列（GenBank登录号MK172784）鉴定为头状蜜环菌（A. cepistipes）。DNA提取、PCR和tef1测序遵循埃利亚斯 - 罗曼等人（2018年）的方案。在九次体细胞不亲和性试验重复（18个测试菌株代表六种北美蜜环菌属物种）中，该菌株与三个头状蜜环菌测试菌株表现出高种内相容性（无色拮抗）（78%），但与该地区出现的其他蜜环菌属物种相容性较低（0 - 33%）。菌株WV - ARR - 3产生的tef1序列与头状蜜环菌的序列一致性为99%（GenBank登录号JF313115和JF313121）。从俄勒冈州门罗东北约8公里处（北纬44°21'47.57"，西经123°13'14.415"）沿威拉米特河、在收集WV - ARR - 3的麦肯齐河支流下游的河岸林分中一棵成熟的毛果杨上采集到第二个序列几乎相同的菌株（WV - ARR - 1；GenBank登录号MK172783）。头状蜜环菌已在美国华盛顿州的红桤木（Alnus rubra）上被报道（巴尼克等人，1996年）以及在加拿大不列颠哥伦比亚省的阔叶树上被报道（艾伦等人，1996年）。它通常被认为是太平洋西北地区阔叶树上的一种弱病原菌，但在欧洲它也与针叶树和落叶树的致病性有关（例如，利吉斯等人，2005年）。然而，最近的一项系统发育研究表明，北美头状蜜环菌在系统发育上与欧亚头状蜜环菌不同（克洛普芬斯坦等人，2017年），但需要进一步研究来确定北美头状蜜环菌的正式分类地位。据我们所知，头状蜜环菌此前在美国毛果杨上未得到确认，也未在北美正式报道为任何杨树物种的病原菌。需要持续研究以确定头状蜜环菌在太平洋西北地区河岸森林中的分布、寄主范围和生态作用，同时监测其在气候变化下的种群情况。
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