Hemlock dwarf mistletoe
Damage, symptoms and biology
Heavy infections reduce wood quality, diameter and height growth, and sometimes result in the death of the tree. Dead tissues resulting from the parasitic action of the dwarf mistletoe plant provide entrance points for stain and decay producing fungi. Infected branches frequently break due to decay or broom size, presenting a hazard in high-use recreational sites.
Conspicuous witches' broom symptoms caused by localized branch proliferation are associated with most mistletoe species. The size and extent ofthe witches' brooms vary among hosts. Where witches' brooms are observed, branches should be checked for the presence of aerial dwarf mistletoe shoots to distinguish these symptoms from symptoms caused by other pathogens (e.g., Elytroderma needle cast) or physiological disorders. Branches and stems are often swollen at the site of dwarf mistletoe infections. Aerial shoots of dwarf mistletoe vary in size (5 to 8 cm high), colour (usually greenish-yellow), and pattern of branching. For example, Douglas-fir dwarf mistletoe plants are often small and inconspicuous, larch dwarf mistletoe shoots are purple or green, and those of lodge pole pine dwarf mistletoe are arranged in a whorled pattern, distinguishing them from all other species. Basal cups remain after aerial shoots are shed in the fall. Dwarf mistletoe witches' brooms develop from either systemic infections or as a result of discrete, localized infections. On witches' brooms formed from systemic infections, aerial shoots, and basal cups are found near the tips of branches. On local-infection brooms, these structures are only found near the original site of infection.
Since the external structures of dwarf mistletoe are generally not visible for 2 to 3 years after infection, surveys for dwarf mistletoe must consider these latent infections. The aerial shoots of dwarf mistletoe plant are sometimes attacked by insects and fungi. A common fungal parasite, Wallrothiella arceuthobium (Peck) Sacc., produces clusters of black fruiting bodies at the tips of the female flowers, inhibiting the development of the fruit and seed.
The species has two subspecies: tsugense on western hemlock and mertensiana on mountain hemlock.