Comparative susceptibility of conifers to knobcone pine dwarf mistletoe

Seventeen mixed conifer stands infested with knobcone pine dwarf mistletoe (Arceuthobium siskiyouense) were sampled to evaluate host susceptibility to this parasitic plant. Temporary circular plots (radius 6 m) were established around dominant, severely infected knobcone pines (Pinus attenuata). Within plots, species, diameter at breast height, and dwarf mistletoe rating (6-class system) were determined for each live tree taller than 1.4 m. More than 2000 trees were sampled in 140 plots. Based on the incidence of infection, tree species were assigned to host-susceptibility classes. Although Jeffrey pine (Pinus jeffreyi) and shore pine (Pinus contorta var. contorta) have been classified as rare hosts of knobcone pine dwarf mistletoe, my results suggest that Jeffrey pine should be classified as a principal host and shore pine should be classified as an occasional host. No infection was observed on western white pine (Pinus monticola), Douglas-fir (Pseudotsuga menziesii), or incense-cedar (Calocedrus decurrens), indicating that these species should be classified as immune.