Arceuthobium vaginatum (Willd.) Presl subsp. vaginatum (family Viscaceae) is the most widespread and common dwarf mistletoe in Mexico (2). Although most dwarf mistletoes are considered to be relatively host-specific parasites, this species has the broadest host range found in the genus. It has been reported to infect 13 species of pines (Pinus spp., family Pinaceae) (2). Pinus pseudostrobus Lindl. is a common pine within the geographic range of A. vaginatum and has been reported as possibly being immune to this mistletoe (2). However, we have found a location in the Sierra Madre Oriental, Nuevo Leon, Mexico where A. vaginatum subsp. vaginatum is severely parasitizing P. pseudostrobus. The stand of infected P. pseudostrobus is located approximately 3 km east of Laguna de Sanchez(25°19′42″N, 100°15′45″W, elevation 1,950 m). Several hundred P. pseudostrobus are infected at this location; several trees in the stand have one or more dwarf mistletoe infections on nearly every branch and many trees have bole infections. P. pseudostrobus is the only pine growing at this locality, and the extent of infection on this pine clearly indicates it is highly susceptible to A. vaginatum subsp. vaginatum. Hawksworth and Wiens (2) based their tentative classification of P. pseudostrobus as immune to A. vaginatum subsp. vaginatum on observations of uninfected P. pseudostrobus growing near severely infected pines in central Mexico. The discrepancy between the susceptibility of P. pseudostrobus in central Mexico and in Nuevo Leon may be related to the different taxonomic classifications afforded these populations by different pine taxonomists. For example, Perry (3) considers the populations of P. pseudostrobus growing in Nuevo Leon to represent P. pseudostrobus forma megacarpa Loock, while Farjon and Styles (1) treat these populations as typical P. pseudostrobus. Whether the high level of susceptibility of the P. pseudostrobus population near Laguna de Sanchez indicates these populations are taxonomically distinct from typical P. pseudostrobus needs further study, but the severe infection we observed in Nuevo Leon clearly demonstrates that P. pseudostrobus should be reclassified as a principal host of A. vaginatum subsp. vaginatum in northeastern Mexico. Specimens of A. vaginatum subsp. vaginatum on P. pseudostrobus have been deposited at the Deaver Herbarium, Northern Arizona University, Flagstaff (Accession No. 76455). To our knowledge, this is the first report of A. vaginatum subsp. vaginatum on P. pseudostrobus. It should also be noted that the population of A. vaginatum subsp. vaginatum near Laguna de Sanchez is 150 m below the lower elevation limit previously reported for this dwarf mistletoe in Mexico (2).
References: (1) A. Farjon and B. Styles. Pinus (Pinaceae). Flora Neotropica, Monogr. 75. NY Bot. Gard., 1997. (2) F. Hawksworth and D. Wiens. Dwarf mistletoes: Biology, pathology, and systematics. USDA For. Serv. Agric. Handb. 709, 1996. (3) J. P. Perry. The Pines of Mexico and Central America. Timber Press, Portland, OR, 1991.