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Southwestern Dwarf Mistletoe, Arceuthobium vaginatum subsp. cryptopodum, Found Parasitizing Picea pungens in Colorado

Southwestern dwarf mistletoe (Arceuthobium vaginatum (Willd.) Presl subsp. cryptopodum (Engelm.) Hawksw. & Wiens, Viscaceae) severely parasitizes several species of pines (Pinus spp., family Pinaceae) in Colorado, Utah, Arizona, New Mexico, and northern Mexico, but it has not been reported to parasitize any species of spruce (Picea, family Pinaceae) (1). However, in June 2004, this dwarf mistletoe was observed parasitizing blue spruce (Picea pungens Engelm.) in the Black Forest north of Colorado Springs, CO (39°02.118′N, 104°36.028′W, elevation 2,250 m). The infected blue spruce was planted as an ornamental approximately 4 m from a 16-m-high ponderosa pine (Pinus ponderosa Douglas ex Lawson & C. Lawson) severely infected with southwestern dwarf mistletoe. Mature dwarf mistletoe shoots were produced on five infected branches of the blue spruce. These shoots were compared with a morphological description of southwestern dwarf mistletoe (1) and this was sufficient for a positive identification of the dwarf mistletoe. The other dwarf mistletoes reported to infect blue spruce are Arceuthobium microcarpum (Engelm.) Hawksw. & Wiens, A. americanum Nutt. ex Engelm., and A. douglasii Engelm.; these are all morphologically distinct from southwestern dwarf mistletoe (1). Three of the infected branches formed small (less than 0.3 m in diameter), nonsystemic witches' brooms. All of the infections on the 6-m-high blue spruce were higher than 1 m on the tree. Thus, it is likely that the spruce was infected after it was transplanted. Three other blue spruces were also located within 4 m of the infected ponderosa pine, but these trees were not infected. To our knowledge, this is the first report of southwestern dwarf mistletoe parasitizing blue spruce and the first report of this dwarf mistletoe on Picea spp. Voucher specimens have been deposited in the Deaver Herbarium, Northern Arizona University, Flagstaff (Accession No. 73959).

References: (1) F. Hawksworth and D. Wiens. Dwarf mistletoes: Biology, pathology, and systematics. USDA For. Serv. Agric. Handb. 709, 1996.