Susceptibility of sugar pine, Shasta red fir and Sierra lodgepole pine to mountain hemlock dwarf mistletoe (Arceuthobiumtsugensesubsp.mertensianae, Viscaceae) in south central Oregon
Authors: Oblinger Brent W. 
Published: May 2021

AbstractIn southern Oregon and northern California, mountain hemlock dwarf mistletoe (MHDM;Arceuthobium tsugensesubsp.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 magnificavar.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.