Arceuthobium

The genus Arceuthobium, commonly called Dwarf Mistletoes, are a genus of 42 species of parasitic plants that parasitize members of Pinaceae and Cupressaceae in North America, Central America, Asia and Africa. Of the 42 species, 39 and 21 are endemic to North America and the United States, respectively. They all have very reduced shoots and leaves (mostly reduced to scales) with the bulk of the plant living under the host's bark. Recently the number of species within the genus has been reduced to 26 as a result of more detailed phylogenetic analysis^[1].

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In the spruce, fir and pine forests of western North America, the various species of dwarf mistletoe are considered pests as they distort the branches and shoots of the trees that they infect, often causing witch's brooms. In addition, severe infection can cause a reduction in tree growth, increased tree mortality, reduced seed and cone development, reduced wood quality, and an increased susceptibility of host trees to pathogen and insect attack. Most of the commercially important conifers in western North America are parasitized by one or more dwarf mistletoes.

They are dioecious, individual plants being either male or female. The fruits that follow fertilization are unusual in building hydrostatic pressure internally when ripe and shooting the single sticky seed up to nearly 50mph, an example of rapid plant movement. The seeds are enveloped in a hygroscopic, glue-like substance called viscin. Many fail to land on a suitable host's shoot, but just as many succeed, and in this way they are spread through the forests as a pest front. The spread of dwarf mistletoes in forest stands is greatest from the overstory to the understory (gravity). Advantageous stand conditions for the spread of dwarf mistletoes include an uneven-aged stand structure with severely infected-hosts in dominant and codominant crown classes, species composition dominated by the primary host, and tree densities that are between 175 - 500 trees/ha.

There are also a number of species from Europe and Asia including one of the smallest in the genus, A. minutissimum that lives on its host, Pinus wallichiana in the Himalaya.

Species

• Arceuthobium abietinum Engelm. ex Munz
• Arceuthobium americanum Nutt. ex Engelm.
• Arceuthobium apachecum Hawksworth & Wiens
• Arceuthobium blumeri A. Nels.
• Arceuthobium californicum Hawksworth & Wiens
• Arceuthobium campylopodum Engelm.
• Arceuthobium cyanocarpum (A. Nels. ex Rydb.) A. Nels.
• Arceuthobium divaricatum Engelm.
• Arceuthobium douglasii Engelm.
• Arceuthobium gillii Hawksworth & Wiens
• Arceuthobium globosum Hawksw. & Wiens
• Arceuthobium laricis (Piper) St. John
• Arceuthobium littorum Hawksworth, Wiens & Nickrent
• Arceuthobium microcarpum (Engelm.) Hawksworth & Wiens
• Arceuthobium minutissimum
• Arceuthobium monticola Hawksworth, Wiens & Nickrent
• Arceuthobium occidentale Engelm.
• Arceuthobium oxycedri
• Arceuthobium pusillum Peck
• Arceuthobium siskiyouense Hawksworth, Wiens & Nickrent
• Arceuthobium tsugense (Rosendahl) G.N. Jones
• Arceuthobium vaginatum (Humb. & Bonpl. ex Willd.) J. Presl (Syn. Viscum vaginatum Humb. & Bonpl. ex Willd.)

References and external links

• Hawksworth, F. G., & Wiens, D. (1996). Dwarf Mistletoes: Biology, Pathology, and Systematics. USDA Forest Service, Agriculture Handbook 709.

• Kenaley, S.C., R.L. Mathiasen, & C.M. Daugherty. 2006. Selection of dwarf mistletoe-infected ponderosa pines by Ips species (Coleoptera: Scolytidae) in northern Arizona. WNAN 66:279-284.

• Mathiasen, R.L. 1996. Dwarf mistletoes in forest canopies. Northwest. Sci. 70:61-71.

• Maleshoots of A. campylopodum in California
• Fruiting plant of A. campylopodum on Pine shoot
• PLANTS profile for Arceuthobium (United States) including range maps.