Comparative susceptibility of subalpine firs to Douglas-fir dwarf mistletoe

Subalpine fir (Abies Lasiocarpa (Hook.) Nutt.) and Rocky Mountain subalpine fir (Abies bifolia A. Murr.) are more susceptible to infection by Douglas-fir dwarf mistletoe (Arceuthobium douglasii Engelm.) than previously reported. Temporary circular plots (6-m radius) were established around dominant, severely infected Douglas-fir (Pseudotsuga menziesii (Mirb.) Franco). 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. Sixty-six percent and 15% of the subalpine fir and Rocky Mountain subalpine fir, respectively, sampled near large, severely infected Douglas-fir were infected. In contrast, over 90% of the Douglas-fir in the same plots were infected. Based on these results, subalpine fir is classified as a secondary host of Douglas-fir dwarf mistletoe and Rocky Mountain subalpine fir is classified as an occasional host. Variation in the infection of subalpine fir by Douglas-fir dwarf mistletoe in relation to the taxonomic status of these hosts is discussed.