Western spruce dwarf mistletoe Western spruce dwarf mistletoe

Arceuthobium douglasii Engelmann var. microcarpum Engelmann in J. T. Rothrock, Rep. U.S. Geogr. Surv., Wheeler, 253. 1879; A. microcarpum (Engelmann) Hawksworth & Wiens

Plants forming witches' brooms. Stems yellowish green, green, orange, red, maroon, or purple, 5(–11) cm; third internode 5–9.3(–16) × 1–1.5(–2) mm, dominant shoot 1.5–3 mm diam. at base. Staminate flowers 2.3 mm diam.; petals 3(–4). Fruits 3.5 × 2 mm.

Flowering (Jul–)Aug–Sep; fruiting (Aug–)Sep(–Oct). Coniferous forests, especially with blue or Engelmann spruce; 2400–3200 m; Ariz., N.Mex.

Meiosis occurs in July, with fruits maturing 12 to 13 months after pollination.

Subspecies microcarpum is a serious pathogen on its principal hosts, Picea engelmannii and P. pungens, as well as on Pinus aristata in northern Arizona. Rare hosts include Abies lasiocarpa and Pinus strobiformis. Interestingly, this mistletoe is not found in the central Rocky Mountains where its principal hosts are most abundant.