The genus ''Arceuthobium'', commonly called dwarf

mistletoe Mistletoe is the common name for obligate hemiparasitic plants in the order Santalales. They are attached to their host tree or shrub by a structure called the haustorium, through which they extract water and nutrients from the host plant. ...

s, is a genus of 26 species of parasitic plants that parasitize members of Pinaceae and

Cupressaceae Cupressaceae is a conifer family, the cypress family, with worldwide distribution. The family includes 27–30 genera (17 monotypic), which include the junipers and redwoods, with about 130–140 species in total. They are monoecious, subdio ...

North America North America is a continent in the Northern Hemisphere and almost entirely within the Western Hemisphere. It is bordered to the north by the Arctic Ocean, to the east by the Atlantic Ocean, to the southeast by South America and the Car ...

, Central America, Asia, Europe, and Africa. Of the 42 species that have been recognized, 39 and 21 of these are endemic to North America and the United States, respectively. They all have very reduced shoots and

leaves A leaf (plural, : leaves) is any of the principal appendages of a vascular plant plant stem, stem, usually borne laterally aboveground and specialized for photosynthesis. Leaves are collectively called foliage, as in "autumn foliage", wh ...

(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 genetic analysis.

Description

They are

dioecious Dioecy (; ; adj. dioecious , ) is a characteristic of a species, meaning that it has distinct individual organisms (unisexual) that produce male or female gametes, either directly (in animals) or indirectly (in seed plants). Dioecious reproductio ...

, individual plants being either male or female. The fruit is unusual in that it builds up hydrostatic pressure internally when ripe and shoots the single sticky seed up to speeds nearly , an example of rapid plant movement. The lodgepole pine dwarf mistletoe,'' Arceuthobium americanum, has been found to explosively-disperse its seeds through thermogenesis.
Rolena A.J. deBruyn, Mark Paetkau, Kelly A. Ross, David V. Godfrey & Cynthia Ross Friedman. Thermogenesis-triggered seed dispersal in dwarf mistletoe. Dwarf mistletoe seeds are enveloped in a hygroscopic, glue-like substance called viscin. Many fail to land on a suitable host's shoot, but some 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, owing to gravity. Advantageous stand conditions for the spread of the parasite 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 between 175 and 500 trees/ha. There are also several species from Europe and Asia including one of the smallest in the genus, ''A. minutissimum'' that lives on its host, '' Pinus wallichiana'' in the Himalayas.

Effects of parasitism

In western forest ecosystems of

, numerous dwarf mistletoe species are considered to be serious forest-borne disease agents. Severe dwarf mistletoe infection can result in a reduction in tree growth, premature tree mortality, reduced seed and cone development, and reduced wood quality, and increases the susceptibility of the host tree to pathogen and/or insect attack. Most of the commercially important conifers in western North America are parasitized by one or more dwarf mistletoes. The interaction between dwarf mistletoes and their host can be generalized as a source-to-sink relationship. Dwarf mistletoes derive the majority of their nutrition from the host’s vascular tissues. Dwarf mistletoes have a root-like endophytic system, composed of primary and secondary haustoria, which invade, but do not injure, both the xylem and phloem of the host. Because this root-like endophytic system is not soilborne, dwarf mistletoes are dependent solely on their host for water. Along a xylem-to-xylem link, dwarf mistletoes draw water from their host using the difference in water potential between parasite and host. The greater transpiration rate of the dwarf mistletoe produces a lower water potential, allowing water to flow from host to parasite. The water gradient or transpiration stream is consistently maintained, even when the host is under moderate water deficit. In addition to host-water dependence, dwarf mistletoes must acquire carbohydrate and mineral nutrition from their hosts. Dwarf mistletoes have both

chlorophyll Chlorophyll (also chlorophyl) is any of several related green pigments found in cyanobacteria and in the chloroplasts of algae and plants. Its name is derived from the Greek words , ("pale green") and , ("leaf"). Chlorophyll allow plants to a ...

a and chlorophyll b and the necessary mechanisms for photosynthesis, but chlorophyll concentrations in dwarf mistletoes are approximately 1/5 to 1/10 of those found in their host’s foliage, and dwarf mistletoes have low photosynthetic rates as measured by the rate of

carbon fixation Biological carbon fixation or сarbon assimilation is the process by which inorganic carbon (particularly in the form of carbon dioxide) is converted to organic compounds by living organisms. The compounds are then used to store energy and as ...

. The principal carbohydrate transported from the host to dwarf mistletoe is

sucrose Sucrose, a disaccharide, is a sugar composed of glucose and fructose subunits. It is produced naturally in plants and is the main constituent of white sugar. It has the molecular formula . For human consumption, sucrose is extracted and refined ...

. Because dwarf mistletoes are phloem-deficient, they draw carbohydrates from their hosts by connections to the host phloem and ray

parenchyma Parenchyma () is the bulk of functional substance in an animal organ or structure such as a tumour. In zoology it is the name for the tissue that fills the interior of flatworms. Etymology The term ''parenchyma'' is New Latin from the word π ...

. The rate of carbohydrate transport varies by season, but dwarf mistletoes continuously draw carbohydrates from their hosts throughout the year.

Dwarf mistletoe rating system

A standardized system called the Hawksworth 6-class dwarf mistletoe rating (DMR) system has been devised to determine how much dwarf mistletoe has infected a tree. To use this system, the living part of the tree crown needs to be broken up into 3 sections, (top, middle, bottom). Each section is then rated either 0, 1, or 2 with a score of 0 being assigned for a uninfected section, 1 for light infection with fewer than half of the branches infected, or 2 for heavy infection with over half of the branches infected. The 3 numbers are then added together to give the total rating for the tree.

Species

Arceuthobium occidentale on Pinus sabiniana

* ''

Arceuthobium abietinum ''Arceuthobium abietinum'' is a species of dwarf mistletoe known as fir dwarf mistletoe. It is native to western North America from Washington to New Mexico to northern Mexico, where it lives in coniferous forests as a parasite on various species ...

'' Engelm. ex Munz * '' Arceuthobium americanum'' Nutt. ex Engelm. * ''

Arceuthobium apachecum ''Arceuthobium apachecum'', called the "Apache dwarf mistletoe," is a parasitic plant found on the branches of pine trees in Arizona, New Mexico and Coahuila Coahuila (), formally Coahuila de Zaragoza (), officially the Free and Sovereign ...

'' Hawksworth & Wiens * '' Arceuthobium azoricum'' Hawksw. & Wiens * ''

Arceuthobium blumeri The genus ''Arceuthobium'', commonly called dwarf mistletoes, is a genus of 26 species of parasitic plants that parasitize members of Pinaceae and Cupressaceae in North America, Central America, Asia, Europe, and Africa. Of the 42 species ...

'' A. Nels. * '' Arceuthobium californicum'' Hawksworth & Wiens * ''

Arceuthobium campylopodum ''Arceuthobium campylopodum'' is a species of dwarf mistletoe known as western dwarf mistletoe. It is native to the low to moderate elevation coniferous forests of western North America. It is a common parasite of several species of pine tree, in ...

'' Engelm. * ''

Arceuthobium cyanocarpum The genus ''Arceuthobium'', commonly called dwarf mistletoes, is a genus of 26 species of parasitic plants that parasitize members of Pinaceae and Cupressaceae in North America, Central America, Asia, Europe, and Africa. Of the 42 species that h ...

'' (A. Nels. ex Rydb.) A. Nels. * ''

Arceuthobium divaricatum ''Arceuthobium divaricatum'' is a species of dwarf mistletoe known as pinyon dwarf mistletoe. It is native to the southwestern United States and Baja California, where it lives in woodlands as a parasite on various species of pine, particularly C ...

'' Engelm. * ''

Arceuthobium douglasii ''Arceuthobium douglasii'' is a species of dwarf mistletoe known as Douglas fir dwarf mistletoe. It is native to western North America from British Columbia to Texas to California, where it lives in forest and woodland as a parasite. It is found ...

'' Engelm. * ''

Arceuthobium gillii ''Arceuthobium gillii'', common name "Chihuahua pine dwarf mistletoe," is a parasitic plant found in Arizona, New Mexico, Chihuahua, Sonora and Sinaloa. It is found mostly on the Chihuahua pine ''Pinus leiophylla'', commonly known as Chihuah ...

'' Hawksworth & Wiens * '' Arceuthobium globosum'' Hawksw. & Wiens * ''

Arceuthobium laricis The genus ''Arceuthobium'', commonly called dwarf mistletoes, is a genus of 26 species of parasitic plants that parasitize members of Pinaceae and Cupressaceae in North America, Central America, Asia, Europe, and Africa. Of the 42 species ...

'' (Piper) St. John * ''

Arceuthobium littorum ''Arceuthobium littorum'' is a species of Arceuthobium, dwarf mistletoe known as coastal dwarf mistletoe. It is Endemism, endemic to the coastline of northern California, where it lives as a parasitic plant, parasite on Bishop Pine and Monterey P ...

'' Hawksworth, Wiens & Nickrent * ''

Arceuthobium microcarpum ''Arceuthobium microcarpum'', called the "western spruce dwarf mistletoe," is a parasitic plant known only from Arizona and New Mexico. It is found mostly on spruce trees (''Picea'' spp.) but also occasionally on Rocky Mountain bristlecone pine ...

'' (Engelm.) Hawksworth & Wiens * '' Arceuthobium minutissimum'' Hook. f. * ''

Arceuthobium monticola ''Arceuthobium monticola'' is a species of dwarf mistletoe known as western white pine dwarf mistletoe. It is endemic to the Klamath Mountains of northern California and southern Oregon, where it lives as a parasite on western white pine tr ...

'' Hawksworth, Wiens & Nickrent * ''

Arceuthobium occidentale The genus ''Arceuthobium'', commonly called dwarf mistletoes, is a genus of 26 species of parasitic plants that parasitize members of Pinaceae and Cupressaceae in North America, Central America, Asia, Europe, and Africa. Of the 42 species ...

'' Engelm. * '' Arceuthobium oxycedri'' (DC.) M.Bieb. * ''

Arceuthobium pusillum ''Arceuthobium pusillum'' is a perennial, obligate parasitic plant in the sandalwood family. Its common names include Dwarf mistletoe or Eastern dwarf mistletoe. It is one of the most widespread dwarf mistletoes within its range which covers t ...

'' Peck * ''

Arceuthobium siskiyouense ''Arceuthobium siskiyouense'' is a species of Arceuthobium, dwarf mistletoe known as knobcone pine dwarf mistletoe. It is Endemism, endemic to the Klamath Mountains of northern California and southern Oregon, where it lives as a parasitic plant, ...

'' Hawksworth, Wiens & Nickrent * '' Arceuthobium tsugense'' (Rosendahl) G.N. Jones * ''

Arceuthobium vaginatum ''Arceuthobium vaginatum'', called the "sheathed dwarf mistletoe" or "southwestern dwarf mistletoe" is a parasitic plant found in the southwestern United States and northwestern and central Mexico. It generally is found on pine (Pinus spp) trees. ...

'' (Humb. & Bonpl. ex Willd.) J. Presl (Syn. ''Viscum vaginatum'' Humb. & Bonpl. ex Willd.)

References

External links

Maleshoots of ''A. campylopodum'' in California

Fruiting plant of ''A. campylopodum'' on Pine shoot

PLANTS profile for Arceuthobium (United States) including range maps.

US forest service data about dwarf mistletoes
{{Taxonbar, from=Q244255 Dioecious plants Epiphytes Parasitic plants Santalales genera

Description

Effects of parasitism

Dwarf mistletoe rating system

Species

References

Further reading

External links