Hemlock dwarf mistletoe, Arceuthobium tsugense

Fig. 153 Aerial shoots. Note spindle-shaped branch swelling.	Fig. 154 Swelling on bole of western hemlock caused by dwarf mistletoe.	Fig. 155 Typical witches' brooms resulting from prolonged infection.

Distribution: Throughout the coastal range of western hemlock. Not found in the interior.

Host Susceptibility:  Western hemlock is highly susceptible. Secondary hosts include mountain hemlock, amabilis fir, sub-alpine fir, shore pine, and Sitka spruce.

Signs & Symptoms: Infection by hemlock dwarf mistletoe causes swelling and abnormal branching, which results in broom formation. Aerial shoots arise from these swellings and produce the flowers and seeds of this parasitic plant. Hemlock dwarf mistletoe aerial shoots are perennial, light green to brown, and 5 to 12 cm long. When aerial shoots fall off, they leave small, circular structures called basal cups, which persist on the host bark for years. Basal cups on fallen brooms or on dead lower branches, are evidence of hemlock dwarf mistletoe in older stands.

Damage: Hemlock dwarf mistletoe is widespread throughout the range of coastal western hemlock. The disease is not found in interior western hemlock stands. Heavily infected trees suffer reduced lumber recovery, reduced height and diameter growth, and occasional mortality. The swellings caused by dwarf mistletoe can create an infection court for decay fungi. The combined effects of the weight of dwarf mistletoe brooms and the structural weakness produced by the swellings often lead to breakage of both branches and stems.

Can Be Confused With:  Mature and over-mature coastal western hemlock trees often have an abundance of gnarled, swollen branches. It is easy to confuse these with the brooms and swellings caused by hemlock dwarf mistletoe. Fallen, swollen branches under such trees should be examined for the presence of basal cups. Confirmation of overstory infection requires evidence from fallen brooms or infections on understory trees.

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BC Ministry of Forests
Forest Practices Branch
P.O. Box 9513 Stn. Prov. Gov.
Victoria, BC
V8W 9C2

Section phone: (250) 387-8739
Section fax: (250) 387-2136

Last updated February 18, 2002