White fir dwarf mistletoe (Arceuthobium abietinum Engelm. ex Munz f. sp. concoloris Hawksw. & Wiens) is a serious and common pathogen of white fir (Abies concolor (Gord. & Glend.) Hildebr.), grand fir (A. grandis (Dougl. ex D. Don) Lindl.), and Low's fir (A. lowiana (Gord.) A. Murr.) in the western United States (1). In August 2002, this dwarf mistletoe was observed parasitizing mountain hemlock (Tsuga mertensiana (Bong.) Carr.) growing among severely infected grand fir near the trailhead to Cabot Lake in the Mount Jefferson Wilderness Area, Oregon at 44°34′27″N, 121°43′43″W, elevation 1,340 m. Only 2 of 27 mountain hemlocks observed in this area were infected. One tree had four infections, and one tree had two infections. Several fully developed male plants were found on one of the infected branches of mountain hemlock and were morphologically similar to those growing on the nearby grand fir. Other dwarf mistletoes that commonly parasitize mountain hemlock (Arceuthobium tsugense subsp. mertensianae and Arceuthobium laricis) were not observed in the area. In addition, white fir dwarf mistletoe can be distinguished from these mistletoes by its larger, yellowish shoots (1). Specimens of the mistletoe from mountain hemlock have been deposited in the Deaver Herbarium, Northern Arizona University, Flagstaff. To my knowledge, this is the first report of white fir dwarf mistletoe on mountain hemlock (1).
Reference: (1) F. Hawksworth, and D. Wiens. Dwarf mistletoes: biology, pathology, and systematics. USDA Agric. Handb. 709, 1996
