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Although the classification of pinyon dwarf mistletoe (Arceuthobium divaricatum, Viscaceae) has not been controversial to any extent since Engelmann described it in 1878, a recent taxonomic treatment has included this species in western dwarf mistletoe (A. campylopodum). While pinyon dwarf mistletoe is only known to parasitize pinyon pines (Pinus subsection Cembroides), western dwarf mistletoe as it has been known since the late 1800s is a principal parasite of Pinus ponderosa and P. jeffreyi and has never been observed parasitizing pinyon pines. With reservations about the recent classification of pinyon dwarf mistletoe and its treatment under A. campylopodum, we undertook this study to examine in detail the morphological characteristics of pinyon dwarf mistletoe and compare them with those of western dwarf mistletoe. Pinyon and western dwarf mistletoe populations were sampled throughout most of their geographic ranges and morphological traits including plant, flower, fruit, and seed dimensions were measured. Thereafter, we compared morphological characteristics between A. campylopodum and A. divaricatum using univariate and multivariate statistics to determine significant differences among morphologies of both male and female plants. Our analyses clearly demonstrated that pinyon and western dwarf mistletoe are morphologically distinct as originally proposed by G. Engelmann in the late 19th century. Furthermore, the host affinities of the two taxa clearly distinguish them from each other. Therefore, we recommend that A. campylopodum and A. divaricatum continue to be classified as separate species. Morphological differences between these species are summarized and a key is provided for use in their field identification.

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© 2016 Robert Mathiasen

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