Dwarf Mistletoes

Arceuthobium spp.

Dicotyledoneae, Viscaceae, Loranthaceae

Dwarf mistletoes are parasitic flowering plants that grow on stems and branches of living conifers. They depend almost entirely on their hosts for support, water, and nutrition. Both male and female plants are generally produced on the same host, but arising from separate infections. Following fertilization, the female plant produces green to dark brown-purple berries from which the mature seeds are forcibly ejected for distances up to 15 m. The seeds, covered with a sticky, mucilaginous pulp, must germinate on and penetrate the bark of a susceptible host to survive. After penetration, a system of root-like strands and perennial sinkers develops in the inner bark. The sinkers ultimately come in contact with and become imbedded in the woody tissues of the tree causing distortion of the annual rings and swelling of bark and wood tissues. Aerial shoots, buds and flowers usually develop within 3 years of initial infection. Dwarf mistletoe shoots vary in size, for example, those of Douglas-fir dwarf mistletoe are only about 2.5 cm long whereas lodgepole pine dwarf mistletoe may be as long as 12 cm.

Hosts: Most Dwarf_mistletoe_species are host-specific, occurring primarily on one species of conifer. Many Dwarf_mistletoe_species will spread from the preferred hosts to other conifer species when they are growing in close proximity. Western redcedar, yellow cedar, western yew, and juniper appear to be immune from infection but all other native conifers in B.C. are susceptible to attack.

Identification: Conspicuous broom symptoms caused by localized branch proliferation are associated with most mistletoe species. The size and extent of brooms varies among hosts (Figs. 63a-f). Where brooms are observed, branches should be checked for the presence of aerial dwarf mistletoe shoots to distinguish broom 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 (Fig. 63g). Aerial shoots of the dwarf mistletoe plants vary in size (typically 5-8 cm high), colour (usually greenish-yellow), and pattern of branching (Figs. 63h, 63i). For example, Douglas-fir dwarf mistletoe plants are often small and inconspicuous (Fig. 63j), larch dwarf mistletoe shoots are purple or green, and those of lodgepole 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 (Fig. 63j). Mistletoe brooms develop from either systemic infections or as a result of discrete, localized infections. On 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.

Damage: 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 (Fig. 63k).

Remarks: Since the external structures of dwarf mistletoes are generally not visible for 2-3 years after infection, surveys for mistletoe must consider these latent infections. The aerial shoots of the 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. Some scientists treat all dwarf mistletoes found in B.C. as one species, Arceuthobium campylopodum Engelmann in Grey, but this book follows the taxonomic system of Hawksworth and Weins (1995), assigning names according to host-specificity

Dwarf mistletoe species	Hosts	Geographic distribution
Douglas-fir dwarf mistletoe Arceuthobium douglasii Engelmann	Found mainly on Douglas-fir, occasionally on grand fir and Engelmann spruce.	In B.C., Douglas-fir mistletoe is restricted to the southern interior. It does not occur west of the Cascades in B.C., Washington, or Oregon.
Larch dwarf mistletoe Arceuthobium laricis (Piper) St. John	The preferred host is western larch, occasionally found on western white, ponderosa, and lodgepole pine, subalpine and grand fir, and Engelmann spruce.	Larch dwarf mistletoe is found in the southeast interior of the province following the range of western larch.
Lodgepole pine dwarf mistletoe Arceuthobium americanum Nutt. ex Engelmann	Lodgepole pine is the preferred host, occasionally found on ponderosa pine and white and Engelmann spruce.	Lodgepole pine dwarf mistletoe is found throughout the range of lodgepole pine in the interior of B.C.
Hemlock dwarf mistletoe Arceuthobium tsugense (Rosendahl) Jones	Western and mountain hemlock are the preferred hosts, occasionally found on lodgepole pine, amabilis fir, rarely on grand and subalpine fir, Sitka and Englemann spruce, and western white pine.	Hemlock dwarf mistletoe is restricted to the range of coastal western hemlock. It is not found in the interior of B.C.

References:

Van Sickle, G. A. and R. B. Smith. 1978. Dwarf mistletoe controls in British Columbia. In USFS Gen. Tech. Rep. PSW-31. pp. 106-111.

Hawksworth, F. G. and D. Wiens. 1995. Dwarf mistletoe: biology, pathology, and systematics. USDA For. Serv. Agric. Hdbk. No. 450.

Unger, L. 1992. Dwarf mistletoes. Can. For. Serv., Forest Pest Leaf. No. 44. Victoria, B.C.

Figures

Click on any image to see the full size version. Press "Back" on your browser to return to this screen.

Figures 63a-63f: Broom symptoms of Dwarf_mistletoe_species.

Figure 63a: Douglas-fir dwarf mistletoe.

Figure 63b: Douglas-fir dwarf mistletoe

Figure 63c: Larch dwarf mistletoe.

Figure 63d: Lodgepole pine dwarf mistletoe.

Figure 63e: Lodgepole pine dwarf mistletoe.

Figure 63f: Hemlock dwarf mistletoe.

Figure 63g: Stem swelling associated with Lodgepole pine dwarf mistletoe.

Figure 63h: Aerial shoots and berries of Lodgepole pine dwarf mistletoe.

Figure 63i: Aerial shoots and berries of Larch dwarf mistletoe. Note purple colour of shoots.

Figure 63j: Small aerial shoots and basal cups of Douglas-fir dwarf mistletoe.

Figure 63k: Broken branch due to a heavy broom of Hemlock dwarf mistletoe on western hemlock.