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 morphological characters such as plant height (mean 8.4 cm), plant color (yellow-green, green, green-brown, and rarely red-brown), mean diameter of flowers (2.8 mm), and fruit dimensions (5.0 × 3.0 mm). It should be noted that although the dwarf mistletoe parasitizing Durango fir on Cerro Mohinora had been classified as white fir dwarf mistletoe (1), the morphological characters above are slightly different than those reported previously for this mistletoe (1,2). On the basis of the number of infected trees and the light to moderate level of infection observed, Mexican spruce should be tentatively classified as an occasional host of white fir dwarf mistletoe using the host susceptibility classification system proposed by Hawksworth and Wiens (1). Specimens of white fir dwarf mistletoe on Mexican spruce were collected and deposited at the Deaver Herbarium (ASC), Northern Arizona University, Flagstaff (Accession No. 93827). To my knowledge, this is the first report of white fir dwarf mistletoe parasitizing Mexican spruce and the only know instance of white fir dwarf mistletoe parasitizing a species of spruce found in Mexico (1). References: (1) F. Hawksworth and D. Wiens. USDA For. Serv. Agric. Handb. 709, 1996. (2) R. Mathiasen and C. Daugherty. Madroño 56:120, 2009.
Dwarf Mistletoe-Host Interactions in Mixed-Conifer Forests in the Sierra Nevada