The Durangan dwarf mistletoe (Arceuthobium vaginatum subsp. durangense Hawksw. & Wiens, Viscaceae) parasitizes several species of pines (Pinus spp., Pinaceae) in the Mexican states of Durango, Sinaloa, and Jalisco (1,3). This mistletoe has primarily been reported from the western edge of the Sierra Madre Occidental in eastern Sinaloa and western Durango, but its distribution there is not well documented (3). In March 2007, I found Durangan dwarf mistletoe parasitizing Pinus engelmannii Carr., approximately 25 km north of Mexico Route 40 (24°00′51″N, 105°26′48″W, elevation 2,200 m), and on P. cooperi Blanco approximately 90 km north of Route 40 (24°24′40″N, 105°35′26″W, elevation 2,710 m) along the road to San Miguel de las Cruces, Durango. These populations are approximately 50 and 80 km northeast of the closest known population of Durangan dwarf mistletoe west of Buenos Aires along Route 40 in extreme western Durango. Infection of P. engelmannii was severe on 25 trees, but only severe on two trees of P. cooperi. No mortality associated with infection by Durangan dwarf mistletoe was observed at either location. Infection caused large, nonsystemic witches' brooms on P. engelmannii, but no brooms were observed on infected P. cooperi. To my knowledge, this is the first report of Durangan dwarf mistletoe on P. cooperi and P. engelmannii, and the first report of this mistletoe from the central Sierra Madre Occidental (3). Although Hawksworth and Wiens (2,3) treated Durangan dwarf mistletoe as a species (A. durangense Hawksw. & Wiens), I use the earlier classification of Durangan dwarf mistletoe as a subspecies of Mexican dwarf mistletoe (A. vaginatum (Willd.) Presl subsp. vaginatum) (1) because of recent molecular evidence (4) and morphological similarities with Mexican dwarf mistletoe. The principal difference between these mistletoes is that plants of Durangan dwarf mistletoe are bright orange while those of Mexican dwarf mistletoe are dark brown to black (1--3). Specimens of Durangan dwarf mistletoe on Pinus engelmannii and P. cooperi have been deposited at the Deaver Herbarium, Northern Arizona University, Flagstaff (Accession Nos. 76455 and 76456, respectively).
References: (1) F. G. Hawksworth and D. Wiens, Brittonia 17:213, 1965. (2) F. G. Hawksworth and D. Wiens. Phytologia 66:3, 1989. (3) F. G. Hawksworth and D. Wiens. USDA For. Serv. Agric. Handb. 709, 1996. (4) D. L. Nickrent et al. Am. J. Bot. 91:125, 2004.
