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Spread, Intensification, and Upward Advance of Dwarf Mistletoe in Thinned, Young Stands of Western Hemlock in Southeast Alaska. Charles G. Shaw III, Research Plant Pathologist, USDA Forest Service, Rocky Mountain Forest and Range Experiment Station, Fort Collins, CO 80526. Paul E. Hennon, Plant Pathologist, USDA Forest Service, Alaska Region, Juneau 99801. Plant Dis. 75:363-367. Accepted for publication 27 September 1990. This article is in the public domain and not copyrightable. It may be freely reprinted with customary crediting of the source. The American Phytopathological Society, 1991. DOI: 10.1094/PD-75-0363.

Examinations of 206 young western hemlock trees growing beneath residual trees infected with hemlock dwarf mistletoe (Arceuthobium tsugense) in two thinned stands were made from 1981 to 1987 to evaluate disease spread and intensification. The percentage of trees infected increased over this period, as did the number of infections on 79% of the trees that were infected in 1981. However, few of the infected trees had three or more infections and only eight trees had 10 or more infections by 1987. The probability of a tree being infected was significantly greater if the tree was in the understory before logging rather than having become established in the understory after logging. There was no significant orientation by distance or direction of infected trees from the infected residual trees. Infections were significantly concentrated in interior portions of the lower crown. From 1981 to 1987, infected hemlock trees grew an average of two times greater in height than the dwarf mistletoe advanced upward in their crowns. The low number of infections on these trees, their concentration over time in interior portions of the lower crown, the sparsity of witches’-brooms, the rapid increases in tree height after thinning, and the common interspersion of resistant Sitka spruce trees all indicate that there is little likelihood of damaging disease levels developing in thinned stands on high quality sites within the planned 90- to 120-yr rotation.