Arceuthobium (dwarf mistletoes), a well defined but morphologically reduced genus of the family Viscaceae, is parasitic on Pinaceae in the Old and New Worlds and on Cupressaceae in the Old World. Although conifer forests in many parts of the Northern Hemisphere are infested with dwarf mistletoes, those most commonly infested are in western North America and Mexico. In North America, Arceuthobium ranges from central Canada and southeastern Alaska to Honduras. Only A. pusillum occurs in eastern North America, and only A. juniperi-procerae is found in the Southern Hemisphere. Arceuthobium bicarinatum and A. azoricum are restricted to islands (Hispaniola and the Azores, respectively).
In this taxonomic revision, the 46 recognized taxa comprise 42 species—4 with 2 subspecies each, and 1 with 2 formae speciales. Eight species are known in the Old World and thirty-four species occur in the New. Natural hybridization and polyploidy are unknown and have resulted in a relatively clear, dendritic line of evolution. The genus is probably of early Tertiary origin and its closest relative is the genus Notothixos, which has tropical Asian and Australasian distribution. Arceuthobium presumably migrated to the New World before the Miocene Epoch. Intensive adaptive radiation occurred into the Pinacea. Some of the species of Abies, Picea, Tsuga, Larix, and Pseudotsuga and 95% of the species of Pinus are parasitized.
The morphological characteristics that delimit species of Arceuthobium are often cryptic and may be apparent only for short periods of the life cycle. Species and subspecies nonetheless differ in a number of discontinuous variables. Most species are sympatric with other members of the genus somewhere in their distribution and flowering periods often overlap, but species appear to be isolated reproductively.
In addition to systematic and descriptive information for each species, we review ecological relationships, biotic associates, physiology, anatomy, pathogenic effects, and methods of control for the Arceuthobium. Color pictures, distribution maps, and a list of specimens examined are provided.