In southern Oregon and northern California, mountain hemlock dwarf mistletoe (MHDM; Arceuthobium tsugense subsp. mertensianae) commonly parasitizes mountain hemlock (Tsuga mertensiana) in multiple mountain ranges such as the Cascades, Sierra Nevada and Klamath Mountains. However, limited information is available on the susceptibility of sugar pine (Pinus lambertiana), Shasta red fir (Abies magnifica var. shastensis) and lodgepole pine (Pinus contorta) to MHDM. From 2015–2019, I found MHDM on sugar pine at multiple sites in northwestern Klamath County, Oregon. To evaluate the susceptibilities of sugar pine, Shasta red fir and Sierra lodgepole pine to MHDM infection, 41 temporary, fixed-radius plots (area 182.4 m²) were established at five sites in 2018 and 2019 around an overstory tree (western white, sugar and whitebark pines along with one mountain hemlock) severely infected with MHDM in each plot centre. Live trees ≥2.6 cm in diameter at breast height (1.37 m; dbh) were assigned a dwarf mistletoe severity rating (6-class system) and incidence (% infection) of MHDM on live trees at the tree level were used to estimate host susceptibilities. To confirm the identity of the dwarf mistletoe infecting live trees in each plot, dwarf mistletoe specimens were collected from the severely infected tree at the centre of each plot and from all tree species bearing dwarf mistletoe in each plot. Morphological characters were used to identify the dwarf mistletoe plants on each tree species and included plant heights, length of the third internode and plant colour. Dwarf mistletoe plants were collected from each affected tree species, and morphological characteristics of aerial shoots of dwarf mistletoes were examined in the laboratory. Morphological measurements confirmed the dwarf mistletoe infesting each plot was MHDM. Mountain hemlock dwarf mistletoe was found on sugar pine in all 41 plots. At the tree level after pooling larger sugar pine trees from all plots (total of 164 live trees ≥2.6 cm dbh), incidence was 76%. Morphological characteristics of MHDM aerial shoots, such as colour, shoot height and third internode length, were consistent with previous descriptions of MHDM plants. These data indicate that sugar pine should be classified as a secondary host of MHDM in the Oregon East Cascades, and not as a rare host as previously reported. No dwarf mistletoe plants were found on 98 living Shasta red fir trees ≥2.6 cm dbh in 26 plots. Shasta red fir should be classified as immune to MHDM. No MHDM was found on 573 living lodgepole pines of this size in 41 plots, and the tree species should be classified as immune to MHDM. Western white (Pinus monticola) and whitebark (Pinus albicaulis) pines were severely parasitized by MHDM with 92% and 100% incidence, respectively. However, because the sample sizes for these white pines were relatively small, additional work is needed to evaluate the susceptibility of these pines to MHDM.

中文翻译：

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俄勒冈州中南部糖松、沙斯塔红冷杉和塞拉罗克松对山铁杉矮槲寄生（Arceuthobium tsugense subsp. mertensianae，Viscaceae）的敏感性

在俄勒冈州南部和加利福尼亚州北部，山芹矮槲寄生（MHDM;油杉寄生属tsugense。亚mertensianae）一般寄生山铁杉（铁杉mertensiana）多山脉，如瀑布，内华达山脉和克拉马斯山脉。然而，关于糖松（Pinuslambertiana）、沙斯塔红冷杉（Abies magnifica var. shastensis）和黑松（Pinus contorta）的敏感性信息有限) 到 MHDM。从 2015 年到 2019 年，我在俄勒冈州克拉马斯县西北部的多个地点的糖松上发现了 MHDM。为了评估糖松、沙斯塔红杉和塞拉黑松对 MHDM 感染的敏感性，41 个临时固定半径地块（面积 182.4 m ²) 于 2018 年和 2019 年在五个地点围绕一棵上层树（西部白松、糖松和白皮松以及一个山铁杉）在每个地块中心严重感染了 MHDM。胸高直径≥2.6 cm (1.37 m; dbh) 的活树被指定为矮槲寄生严重度等级（6 级系统），并使用树级活树上的 MHDM 发生率（感染百分比）来估计寄主易感性. 为了确认每个地块中感染活树的矮槲寄生的身份，从每个地块中心的严重感染的树和每个地块中所有带有矮槲寄生的树种中收集了矮槲寄生标本。形态特征用于识别每个树种上的矮化槲寄生植物，包括植物高度、第三节间长度和植物颜色。从每个受影响的树种中收集矮槲寄生植物，并在实验室检查矮槲寄生气生枝条的形态特征。形态学测量证实，侵染每个地块的矮槲寄生是 MHDM。在所有 41 个地块的糖松上都发现了山铁杉矮槲寄生。在从所有地块（总共 164 棵活树≥2.6 cm dbh）汇集较大的糖松树后，在树水平上，发生率为 76%。MHDM 气生枝条的形态特征，如颜色、枝条高度和第三节间长度，与先前对 MHDM 植物的描述一致。这些数据表明，糖松应该被归类为俄勒冈东喀斯喀特地区 MHDM 的次要宿主，而不是之前报道的稀有宿主。在 26 个地块中的 98 棵活体沙斯塔红杉树 ≥2.6 cm dbh 上没有发现矮化槲寄生植物。沙斯塔红杉应归类为对 MHDM 免疫。在 41 个地块中的 573 株这种大小的活体黑松中未发现 MHDM，该树种应归类为对 MHDM 免疫。西部白（Pinus monticola ) 和白皮松 ( Pinus albicaulis ) 被 MHDM 严重寄生，发病率分别为 92% 和 100%。然而，由于这些白松树的样本量相对较小，因此需要进行额外的工作来评估这些松树对 MHDM 的敏感性。