Susceptibility of Conifers to Three Dwarf Mistletoes in the Klamath-Siskiyou Mountains

Dwarf mistletoes (Arceuthobium spp., Viscaceae) are parasitic flowering plants that infect members of the Pinaceae family in the western United States. This article reports additional host susceptibility data for three dwarf mistletoes found in the Klamath-Siskiyou Mountains of southwestern Oregon and northwestern California. Three mixed conifer stands, each infested with either mountain hemlock dwarf mistletoe, western white pine dwarf mistletoe, or Wiens' dwarf mistletoe (nine stands total) were sampled to evaluate the susceptibility of conifers to these parasites. At each of the study sites, 10–20 temporary circular plots with a 6-m radius (0.012 ha) were established around large, severely infected trees. Within plots, species, dbh, and dwarf mistletoe rating (six-class system) were determined for each live tree. On the basis of the incidence of infection, conifers were assigned to host susceptibility classes. Western white pine and mountain hemlock were principal hosts of western white pine and mountain hemlock dwarf mistletoes, respectively. Brewer spruce and red fir were principal hosts of Wiens' dwarf mistletoe. Other conifers sampled were less susceptible to these mistletoes. This information can be used by forest managers to mitigate the damage associated with infestations of these dwarf mistletoes in mixed conifer forests of the Klamath-Siskiyou Mountains.