Young red firs (Abies magnifica A. Murr.) and white firs (A. concolor [Gord. & Glend.] Lindl. ex Hildebr.) on the Stanislaus National Forest, California, were inoculated with seeds of dwarf mistletoe (Arceuthobium abietinum) for 5 successive years. Only 3 to 4 percent of about 7000 seeds placed on branches resulted in infections. Second-generation infections developed and populations of the parasite built up on some trees but not on others after 17 years. Variables that appeared to regulate population increases included an 8- to 9-year average between inoculation and fruiting of plants, low rate of fruit production among female plants, low proportion of plants producing abundant fruit (100 or more/year), and irregular production of fruit on plants over the years, Death of infected branches also helped keep populations of the parasite in check. In vigorous, well-managed stands of young firs, dwarf mistletoe populations may not build up rapidly enough to result in serious losses.

Hemlock dwarf mistletoe (HDM) (¿Arceuthobium tsugense¿) is a small, inconspicuous parasite that has significant effects on tree growth and stand structure in coastal forest ecosystems of west. N. Amer. Previous clearcut harvesting of areas that removed all infected trees, and forestry practices that established even-aged stands of trees effectively prevented HDM impacts. However, current forest practices that restrict clearcut harvesting to small openings, and retain live trees to preserve attributes of old-growth forests create conditions that are favorable for enhanced seed production by HDM, early spread of the HDM to infect young trees, and, increased growth impacts to residual trees. More info. is needed on the biology and impacts of HDM. Illustrations.