Arceuthobium americanum

American dwarf mistletoe

Santalaceae

Forest Insect & Disease, USDA

The Basics

Lodgepole pine dwarf mistletoe (Arceuthobium americanum) is a native, parasitic, seed plant that occurs essentially through-out the range of lodgepole pine in North America...The parasite gathers nourishment through the network of absorbing strands within the pine's cortex and xylem.

Dwarf mistletoe development depends directly on the vigor of the host tree: the more vigorous the tree, the more vigorous the mistletoe. On good sites with vigorous hosts, the proportion of trees infected is higher than on poor sites, although the effects of the parasite are less on better sites.

Clearcutting is the best way to control dwarf mistletoe in mature lodgepole pine. All infected trees should be cut, or the sanitation value of the operation will be lost...Even where stands are properly clearcut, some infection will develop in the regeneration bordering infected areas. (Forest Insect & Disease)

Identification

Dwarf mistletoe is a parasitic seed plant. On the host tree's stem and branches, it produces slender, leafless, jointed shoots, which are olive-green to yellow in color.

Recently infested stands show few abnormalities except swellings and inconspicuous dwarf mistletoe shoots on branches and main stems.

Where the parasite has been present for a long time, the stand will have several groups of heavily damaged trees surrounded by increasingly healthier zones of trees. Affected trees are characterized by abnormally tufted branches. These growths, which are caused by the dwarf mistletoe, are called witches' brooms. Brooms of another type-stimulation brooms-are frequently mistaken for those caused by dwarf mistletoe. Stimulation brooms are usually denser than dwarf mistletoe brooms. They occur in formerly suppressed trees or trees with dead or broken-out tops and are most common in residual trees left in cutover areas. (Forest Insect & Disease)

Threats

In the United States, the principal host of Arceuthobium americanum is lodgepole pine. It is occasionally found on Jeffrey, limber, and ponderosa pines. Engelmann spruce, blue spruce, whitebark pine, and Rocky Mountain bristlecone pine may sometimes be infected. In Canada, the principal hosts are lodgepole pine in British Columbia and Alberta and jack pine from Alberta to Ontario.

Recently infested stands show few abnormalities except swellings and inconspicuous dwarf mistletoe shoots on branches and main stems. Where the parasite has been present for a long time, the stand will have several groups of heavily damaged trees surrounded by increasingly healthier zones of trees. Affected trees are characterized by abnormally tufted branches. These growths, which are caused by the dwarf mistletoe, are called witches' brooms. Brooms of another type-stimulation brooms-are frequently mistaken for those caused by dwarf mistletoe. Stimulation brooms are usually denser than dwarf mistletoe brooms. They occur in formerly suppressed trees or trees with dead or broken-out tops and are most common in residual trees left in cutover areas.

Older trees with well-developed, vigorous crowns may not show appreciable effects from the parasite for years after initial infection. As the parasite spreads through the crown, however, the tree's growth slows; eventually the crown dies and then the tree. Insects, particularly secondary bark beetles, frequently invade heavily infected trees and kill them...Dwarf mistletoe also reduces seed production of the host trees and can cause commercially unacceptable deformities such as cankers and knots. Wood quality is also adversely affected. (Forest Insect & Disease)

Reproduction

On the host tree's stem and branches, it produces slender, leafless, jointed shoots, which are olive-green to yellow in color...The shoots bear the flowers and fruits, but synthesize little food...The principal function of these shoots is reproduction. The plants are about half male and half female. Only the female plant bears the fruits that spread the disease.

Each berry-like fruit contains a single seed. At maturity, the elastic outer case of the fruit, which is under high hydrostatic pressure, breaks from its base, contracts violently, and shoots the seed into the air. The seeds travel at speeds of up to 100 km per hour. They can reach distances of up to 9 m , but most seeds fall within 3-5 m of the source tree...A sticky substance (viscin) surrounds each seed and holds it fast to its landing surface. when seeds land on pine needles and the viscous coating is moistened by rain, the seeds slide down the needles.some may be lost, but many are successfully transferred to the twigs. In the spring, the seeds that settled on twigs germinate, establish their root system in the bark, and start new infections. (Forest insect & Disease)

Species Distribution

Lodgepole pine dwarf mistletoe (Arceuthobium americanum Nutt. ex Engelm.) is a native, parasitic, seed plant that occurs essentially through-out the range of lodgepole pine in North America.

In the United States, the principal host of Arceuthobium americanum is lodgepole pine. It is occasionally found on Jeffrey, limber, and ponderosa pines. Engelmann spruce, blue spruce, whitebark pine, and Rocky Mountain bristlecone pine may sometimes be infected. In Canada, the principal hosts are lodgepole pine in British Columbia and Alberta and jack pine from Alberta to Ontario. In British Columbia, the parasite has also been found on shore pine, which is the low-elevation subspecies of lodgepole pine along the Pacific Coast. it has not been reported on this tree in the United States. (Forest Insect & Disease)

Citation

USDA Plant Database http://plants.usda.gov/characteristics.html USDA, NRCS. 2016. The PLANTS Database (http://plants.usda.gov, 4 February 2016). National Plant Data Team, Greensboro, NC 27401-4901 USA.

Flora of North America http://www.efloras.org/flora_page.aspx?flora_id=1 Flora of North America Editorial Committee, eds. 1993+. Flora of North America North of Mexico. 19+ vols. New York and Oxford.

Silvics of North America Burns, R.M., and B.H. Honkala. 1990. Silvics of North America (Volume 1: Conifers, Volume 2: Hardwoods). USDA Forest Service Agricultural Handbook 654.

Intermountain Herbarium http://intermountainbiota.org/portal/collections/harvestparams.php Consortium of Intermountain Herbaria. 2016. http//:intermountainbiota.org/portal/index.php. Accessed on February 04.

Burke Museum Plant Image Collection The plant image collection at the Burke Museum, University of Washington.

Jepson Manual http://ucjeps.berkeley.edu/eflora/ The Jepson Manual: Vascular Plants of California. B.G. Baldwin, D.H. Goldman, D.J. Keil, R. Patterson, T.J. Rosatti, and D.H. Wilken [editors]. 2012. 2nd edition, thoroughly revised and expanded. University of California Press, Berkeley, CA., hardcover; 1600 pages. ISBN-13: 978-0520253124.

USGS Plant Species Range Maps http://esp.cr.usgs.gov/data/little/ Critchfield, W.B., and Little, E.L., Jr., 1966, Geographic distribution of the pines of the world: U.S. Department of Agriculture Miscellaneous Publication 991, p. 1-97. Little, E.L., Jr., 1971-1978, Atlas of United States trees, volume 1,3,13,17, conifers and important hardwoods: U.S. Department of Agriculture Miscellaneous Publications.

Burke Museum of Natural History and Culture
Burke Museum. 2016 [Online]. University of Washington.
Photo credit: G.D. Carr 2015