First Report of White Fir Dwarf Mistletoe (Arceuthobium abietinum f. sp. concoloris) on Mexican Spruce (Picea mexicana) in Northern Mexico

White fir dwarf mistletoe (Arceuthobium abietinum Engelmann ex Munz f. sp. concoloris Hawksw. & Wiens, Viscaceae) severely parasitizes true firs (Abies spp.) from southern Washington to southern California (1). It also occurs in widely isolated populations on white fir (Abies concolor (Gord. & Glend.) Hildebr.) in Nevada, Utah, and Arizona (1). In addition, the two known populations of dwarf mistletoe on Durango fir (Abies durangensis Martínez) in Chihuahua, Mexico (near Yahuirachi and on Cerro Mohinora) have been classified as white fir dwarf mistletoe (1). Although a subspecies of fir dwarf mistletoe (A. abietinum Engelm. ex Munz subsp. wiensii Mathiasen & C. Daugherty) severely parasitizes Brewer spruce (Picea breweriana S. Watson) in northern California and southern Oregon (2), Engelmann spruce (P. engelmannii (Parry) Engelmann), blue spruce (P. pungens Engelm.), and Chihuahua spruce (P. chihuahuana Mart.) have been reported to be immune to infection by white fir dwarf mistletoe in the southwest and Mexico (1). However, in September 2009, white fir dwarf mistletoe was found to be infecting the rare Mexican spruce (P. mexicana Mart.) on Cerro Mohinora in southern Chihuahua, Mexico (25°57′42″N, 107°02′28″W, elevation 3,040 m). Infected Mexican spruces were growing among severely infected Durango firs in a mixed conifer forest of Durango fir, Mexican spruce, Douglas-fir (Pseudotsuga menziesii (Mirb.) Franco), and Mexican white pine (Pinus ayacahuite Ehrenb.). White fir dwarf mistletoe was the only mistletoe present in the forest with the infected Mexican spruces. Only five infected spruces were observed, but some trees had as many as 20 infections. No mortality of Mexican spruce associated with mistletoe infection was observed. Some of the infected spruce branches were producing mature male and female plants with flowers and fruits, respectively. Mistletoe plants collected from Durango fir and Mexican spruce were identical when compared using...