Many young, understory Jeffrey pines (Pinus jefreyi Grev. & Balf.) were found to be infected by western dwarf mistletoe (Arcerrthobium cantpylopodurn Engelm.) on Laguna Mountain, Cleveland National Forest, in southern California. Under heavily infected overstory, about three-fourths of the young pines (about 15 years old on the average) were infected by dwarf mistletoe. Infections per tree averaged about two and a half, two-thirds of these on the bole. Attempts to regenerate Jeffrey pine under an infected overstory in southern California will probably lead to a severely infected and damaged understory. Planting nonhost species or resistant selections of Jeffrey pine would be practical in managing dwarf mistletoe on these highly valuable forest recreational areas.

Western dwarf mistletoe (Arceuthobium campylopodum) is a damaging parasite of Jeffrey pines (Pinus jeffreyi) in southern California. Infected branches that develop into brooms are believed to reduce tlee vigor and increase mortality. Brooms were pruned from Jeffrey pines with varying levels of dwarf mistletoe infection and live crown. Many of the trees showed an increase in crown vigor when compared with unpruned trees after 5 years. Mortality was not reduced by pruning brooms, except during years of below-normal precipitation. Of the trees that died, most had little live crown, heavy dwarf mistletoe infection, and were also attacked by root diseases or insects or both. Reducing the stress of dwarf mistletoe by broom pruning to no less than 30 percent live crown helps to prevent mortality due to root disease and insect attack during years of below-normal precipitation.