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Spread and buildup of dwarf mistletoe, Arceuthobiun abietinum, was studied on inoculated white fir, Abies concolor, and red fir, A. magnifica, in northern Califomia for 23 to 28 years. At the end of these studies (1986), and in the absence of overstory infection, 13 of 23 trees had dwarf mistletoe populations that were the same or smaller than the original populations resulting from inoculation. Mortality of infections was the main factor limiting population increases. Live crown ratio of all trees averaged over 0.8. The average ratio of tree height growth to vertical spread rate of dwarf mistletoe was 11.5 to 1 in white fir and 7 to 1 in red fir in the Sierra Nevada. In the southem Cascades, the average ratio was 1.7 to 1 in red fir. About one fourth of the trees became infected in the bole. Of 14 additional trees infected by lateral spread of the parasite, 13 were within 6 m of the source of infection. Evidence continues to indicate that losses from dwarf mistletoes will be small in well-managed young fir stands free from infected overstory trees and properly spaced to promote good growth.
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.