First Report of Larch Dwarf Mistletoe on Pacific Silver Fir and on Mountain Hemlock in the Cascade Mountains, WA. R. L. Mathiasen, Idaho Department of Lands, 1024 Boyd, Coeur d’Alene 83814 . J. S. Beatty, USDA Forest Service, Forest Insects and Diseases, Columbia River Ranger Station, 31520 SE Woodard Road, Troutdale, OR 97060 and D. M. Hildebrand, USDA Forest Service, Forest Insects and Diseases, P.O. Box 3623, Portland, OR 97208. Plant Dis. 79:1249. Accepted for publication 4 October 1995. Copyright 1995 The American Phytopathological Society. DOI: 10.1094/PD-79-1249C.
In July 1995, larch dwarf mistletoe (Arceuthobium laricis (Piper) St. John) was observed parasitizing Pacific silver fir (Abies amabilis Douglas ex Forbes) growing near infected western larch (Larix occidentalis Nutt.) in several mixed conifer stands on the Gifford Pinchot National Forest approximately 8 miles west of Trout Lake, WA (elevation 1,050 to 1,200 m). Larch dwarf mistletoe was also observed parasitizing mountain hemlock (Tsuga mertensiana (Bong.) Carriere) in the same stands. The number of infected Pacific silver fir was low (approximately 15%), which indicates that it is an occasional host for larch dwarf mistletoe in southern WA based on the host susceptibility classification developed by Hawks-worth and Wiens. Infection on mountain hemlock was more common and intense and it is best classified as a secondary host of larch dwarf mistletoe. Mistletoe shoot production was abundant on both hosts and shoots were morphologically similar to those found on infected western larch in the same area. Specimens of larch dwarf mistletoe on Pacific silver fir and mountain hemlock were collected (Skamania County, T.06N., R.09E., Sec. 17, SWIM, 1,050 m) and have been deposited at the USDA Forest Service Forest Pathology Herbarium in Fort Collins, CO. This is the first report of larch dwarf mistletoe on Pacific silver fir. Larch dwarf mistletoe severely parasitizes mountain hemlock in the Bitterroot Mountains of northern Idaho and western Montana, but this is the first report of this dwarf mistletoe/host combination from the Cascade Mountains, WA.