Bristlecone pine dwarf mistletoe (Arceuthobium microcarpum [Engelm.] Hawksw. & Wiens subsp. aristatae J.M.Scott & Mathiasen) severely
parasitizes Rocky Mountain bristlecone pine (Pinus aristata Engelm.) on the San Francisco Peaks, Arizona. We estimated the current incidence of infection and extent of mortality in a mistletoe-infested stand of bristlecone
pine on Schultz Peak, a southeast subsidiary ridge of the San Francisco Peaks. Dendrochronological techniques were used to compare radial growth between lightly, moderately, and severely infected and noninfected bristlecone pines. We also paired limber pine (Pinus
flexilis James) with noninfected and infected bristlecone pines as another method of estimating the effects of the mistletoe on radial growth. Results suggested that the incidence of infection has increased by approximately 10%, and mortality has increased by approximately
20% since the mid-1970s. Severely infected bristlecone pines had significant growth losses and less annual variation in growth than limber pine or lightly infected and noninfected bristlecone pines. Spatial and temporal trends in mortality of bristlecone pines were measured in two large mortality
locations on Schultz Peak. Cross-dating cores from dead bristlecone pines indicated that mortality had rapidly increased starting in 1996. Years of high mortality were related to both high mountain pine beetle (Dendroctonus ponderosae
Hopkins) activity and drought conditions.