New subspecies of fir dwarf mistletoe (Arceuthobium abietinum: Viscaceae) from the western United States and northern Mexico

Two new subspecies of fir dwarf mistletoe (Arceuthobium abietinum, Viscaceae) are described herein: Arceuthobium abietinum subsp. mathiasenii (Mathiasen’s dwarf mistletoe) and Arceuthobium abietinum subsp. grandae (grand fir dwarf mistletoe). The former dwarf mistletoe—Mathiasen’s dwarf mistletoe—parasitizes Rocky Mountain white fir in Nevada, Utah, and Arizona and Durango fir in northern Mexico; whereas, grand fir dwarf mistletoe parasitizes grand fir as well grand fir × white fir hybrids in California, Oregon and Washington. The recognition of these subspecies and, hence, their classification is based on morphological discontinuities and host range differences when compared to white fir dwarf mistletoe (Arceuthobium abietinum subsp. abietinum), red fir dwarf mistletoe (A. abietinum subsp. magnificae), and Wiens’ dwarf mistletoe (A. abietinum subsp. wiensii). The combined analyses of plant height, third internode length and width, fruit dimensions, staminate spike and petal dimensions, and anther distance to tip, contributed most to the discrimination of subsp. grandae and mathiasenii from subsp. abietinum and magnificae. The flowers of Mathiasen's dwarf mistletoe were larger than all of the other taxa classified under A. abietinum. The shoot color of Mathiasen’s dwarf mistletoe was frequently blue-green, brown, yellow-brown, or red-brown, and plants were often highly glaucous, while the shoot color of the other subspecies were typically yellow-green or yellow, except for Wiens' dwarf mistletoe whose shoots are often green-brown or red-brown. Differences in host specificity and geographic distribution also distinguish the new subspecies from the other taxa of A. abietinum.