Comparative Susceptibility of Corkbark Fir and Douglas-fir to Douglas-fir Dwarf Mistietoe

Abstract Corkbark fir (Abies lasiocarpa var. arizonica [Merriam] Lemm.) is less susceptible to infection by Douglas-fir dwarf mistletoe (Arceuthobium douglasii Engelm.) than previously reported. Infection of corkbark fir was less than 20 percent in nearly all of the mixed conifer stands sampled, regardless of the intensity of infection on Douglas-fir (Pseudotsuga menziesii [Mirb.] Franco), the principal host of A. douglasii. Only 7 percent of the 2,666 live corkbark firs sampled in 47 plots were infected compared to 61 percent infection for 2,106 live Douglas-firs. Infection of most corkbark firs was light to moderate when rated using the 6-class dwarf mistletoe rating system. Infection of corkbark fir increased as infection of Douglas-fir increased, but infection of corkbark fir remained consistently below the levels reached in Douglas-fir. Therefore, the impact of A. douglasii on corkbark fir is not serious on a stand basis, but occasionally large corkbark firs are heavily infected. Control of A. douglasii on corkbark fir need not be a major concern of forest managers in most southwestern mixed conifer forests. Forest Sci. 30:842-847.