Department of Plant Pathology, University of Arizona, Tucson 85721
School of Forestry, P.O. Box 15018, Northern Arizona University, Flagstaff 86011
Phoradendron macrophyllum (Engelm.) Cockerell (Viscaceae) is a common mistletoe found on at least 60 hardwood tree species, including several species of cottonwood (Populus spp.) (1), and is distributed from northern Mexico through Arizona and New Mexico into central California (1). R. Gilbertson recently received a specimen of what appeared to be P. macrophyllum parasitizing a planted aspen (Populus tremuloides Michx.) on the Wear Ranch near Fort Grant, Graham County, AZ (T. 11 S., R. 21 E., Section 11). A large swelling on a branch bearing a few small mistletoe plants measuring less than 5 cm high indicated the infection. Comparison of the mistletoe leaf trichomes with those of P. macrophyllum (trichomes with two branches from a common base) and P. coryae Trel. (trichomes stellate with secondary and tertiary branching) indicated that the specimen was definitely P. macrophyllum (2). To our knowledge, this is the first report of P. macrophyllum, or any other mistletoe, on aspen.
References: (1) R. Scharpf and F. Hawksworth. USDA For. Serv. Pest Leafl. 147, 1974. (2) D. Wiens. Brittonia 16:11, 1964.