Comparative susceptibility of conifers to larch dwarf mistletoe in the pacific northwest

A total of 62 mixed conifer sites in Idaho, Montana, Oregon, and Washington infested with larch dwarf mistletoe (Arceuthobium laricis) were sampled to compare host susceptibility to this parasitic flowering plant. Temporary circular plots (6 m radius) were established around dominant, severely infected, principal, and secondary hosts. Within plots, species, diameter at 1.37 m above the ground, and dwarf mistletoe rating (6 class system) were determined for each live tree over 1.37 m in height. A total of 11,270 trees were sampled in 620 plots. Based on the percentage of infection, species were assigned to host susceptibility classes: western larch is the only principal host of larch dwarf mistletoe; mountain hemlock and lodgepole pine are secondary hosts; subalpine fir and ponderosa pine are occasional hosts; and Engelmann spruce, grand fir, and western white pine are rare hosts. No infection was observed on Douglas-fir, western hemlock or western redcedar. However, based on a report of larch dwarf mistletoe parasitizing western hemlock in Washington, western hemlock must be designated as a rare host. Pacific silver fir and whitebark pine are tentatively classified as occasional hosts.