Susceptibility of Coulter pine (Pinus coulteri) to western dwarf mistletoe (Arceuthobium campylopodum) in southern California

The susceptibility of Coulter pine (Pinus coulteri) to western dwarf mistletoe (Arceuthobium campylopodum) was evaluated in southern California. Eight stands infested with western dwarf mistletoe and dominated by Coulter and Jeffrey pines (Pinus jeffreyi) were sampled. Infection of Coulter pine was evaluated using circular plots established around large, severely infected Jeffrey pines and with rectangular transects. A total of 25 plots and 10 transects were sampled. In each plot and transect, live trees > 5 cm in diameter at breast height (1.4 m above the ground) were assigned a dwarf mistletoe rating (6-class system). Nearly 600 Jeffrey pines and more than 400 Coulter pines were evaluated. Infection of Jeffrey and Coulter pines in the plots and transects averaged 96% and 97%, respectively. These data indicate that Coulter pine should be classified as a principal host of WDM and not as a secondary host as previously reported.