White fir dwarf mistletoe (Arceuthobium abietinum Engelm. ex Munz f. sp. concoloris Hawksw. & Wiens, Viscaceae) is a common parasite of grand fir (Abies grandis (Dougl. ex D. Don) Lindl.) in the Cascade Range and of Sierra white fir (Abies lowiana (Gord. & Glend.) A. Murray) in the Sierra Nevada Mountains (1). It also occurs in isolated populations on Rocky Mountain white fir (Abies concolor (Gord. & Glend.) Hildebr.) in Nevada, Utah, and Arizona (1). In addition, there are two widely separated known populations of white fir dwarf mistletoe on Durango fir (Abies durangensis Mart.) from Chihuahua, Mexico (1,2). The southernmost range of these Mexican populations extends to Cerro Mohinora near Guadalupe y Calvo close to the border with Durango and Sinaloa. In July 2012, white fir dwarf mistletoe was found infecting Durango fir on Cerro Gordo, the highest peak in the state of Durango (Latitude: 23° 12′ 37″ N; Longitude: 104° 56″ 23″ W; elevation 3,060 m). Although there are many populations of Durango fir in Durango between Cerro Gordo and Cerro Mohinora, white fir dwarf mistletoe has never been reported from any of those populations (1). More than 70% of the trees were infected in the stand where the mistletoe was observed on Cerro Gordo, but little mortality of Durango fir was observed (4 trees). The infected Durango firs were growing in a mixed conifer forest of Durango fir, Douglas fir (Pseudotsuga menziesii (Mirb.) Franco), Cooper pine (Pinus cooperi Blanco), Mexican white pine (P. ayacahuite Ehrenb. ex Schltdl.), Durango pine (Pinus durangensis Mart.), and aspen (Populus tremuloides Michx.). There were no pure stands of Durango fir in the area. Infection by white fir dwarf mistletoe was only observed on Durango fir and infection was characterized by the formation of witches' brooms and branch swellings. Mistletoe plants collected from Durango fir on Cerro Gordo were identical to white fir dwarf mistletoe plants found on Cerro Mohinora when compared using morphological characters such as plant height (mean approximately 8 cm), plant color (yellow-green, green, green-brown, and rarely red-brown), mean diameter of flowers (2.8 mm), and mean fruit dimensions (5.0 × 3.0 mm) (2). White fir dwarf mistletoe is relatively host-specific and is the only dwarf mistletoe that has been reported to parasitize Durango fir in Mexico (1). Specimens of white fir dwarf mistletoe from Cerro Gordo were collected and deposited at the Hebario (CIIDIR), Instituto Politecnico Nacional, Durango, Mexico (Accession #40190). To our knowledge, this is the first report of white fir dwarf mistletoe from Durango, Mexico, and extends the known southern range of this mistletoe by approximately 370 km (1).
References: (1) F. Hawksworth and D. Wiens. USDA For. Serv. Agric. Handb. 709, 1996. (2) R. Mathiasen. Plant Dis. 94:635, 2010.