Hemlock dwarf mistletoe
Last updated on January 30, 2025Arceuthobium tsugense
Hemlock dwarf mistletoe is a widespread and damaging parasite in British Columbia's coastal hemlock stands.
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Description
Hemlock dwarf mistletoe is found within the coastal range of western hemlock. It is not found in interior British Columbia.
Infected branches usually occur in dense clumps and feature extreme spindle-shaped swellings and witches broom which generally grow horizontal with little drooping. Mistletoe aerial shoots featuring a square cross-section are found on the swollen portion of branches and are arranged in a fan-like structure. Green berries with sticky seeds are produced.
Host tree species
Western hemlock is the preferred host but sitka and Englemann spruce, grand fir, amabilis fir and western white pine are occasionally attacked. Trees of all ages and sizes are susceptible.
Damage symptoms
Infected branches usually occur in dense clumps and are characterized by extreme spindle-shaped swellings and witches’ brooms.
Severely infected trees show significant growth reduction and prolonged infection can cause mortality. Reduced growth is usually noticeable at 20 years of age and by 40 years of age the volume of moderately or severely infected trees is significantly less than lightly or uninfected trees.
Stem swelling also adversely affects wood quality.
Identification images
Further reading
Read more about hemlock dwarf mistletoe and view images in the Field Guide to Forest Damage in B.C. (PDF, 6.6MB)