Editing Arceuthobium

{{Italic title}}
{{Short description|Genus of mistletoes}}
{{Automatic taxobox
| name = Dwarf mistletoes
| image = Arceuthobium abietinum 1.jpg
| image_caption = ''Arceuthobium abietinum''
| taxon = Arceuthobium
| authority = [[Friedrich August Marschall von Bieberstein|M.Bieb.]]
| subdivision_ranks = Species
| subdivision = See [[list of Arceuthobium species|list of ''Arceuthobium'' species]]
}}

The [[genus]] '''''Arceuthobium''''', commonly called '''dwarf [[mistletoe]]s''', is a genus of 42<ref name="Nick">{{cite journal | last = Nickrent | first = D.L. |author2=M.A. García |author3=M.P. Martín |author4=R.L. Mathiasen | s2cid = 13745441 | year = 2004 | title = A phylogeny of all species of ''Arceuthobium'' (Viscaceae) using nuclear and chloroplast DNA sequences | journal = American Journal of Botany | volume = 91 | pages = 125–138 | doi = 10.3732/ajb.91.1.125 | pmid = 21653369 | issue = 1| doi-access = free | bibcode = 2004AmJB...91..125N }}</ref> species of [[parasitic plant]]s that parasitize members of [[Pinaceae]] and [[Cupressaceae]] in [[North America]], [[Central America]], [[Asia]], [[Europe]], and [[Africa]].<ref name="oneill_rana_2016">{{cite journal|last1=O'Neill|first1=A.R.|last2=Rana|first2=S.K.|date=2019|title=An ethnobotanical analysis of parasitic plants (Parijibi) in the Nepal Himalaya|journal=Journal of Ethnobiology and Ethnomedicine|volume=12|issue=14|pages=14|doi=10.1186/s13002-016-0086-y|pmid=26912113|pmc=4765049 |doi-access=free }}</ref> 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]] (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.<ref name="Nick"/>

==Description==
[[File:Arceuthobium vaginatum subsp. cryptopodum.jpg|thumb|Female plant of ''A. vaginatum'' susbp. ''cryptopodum'' on Ponderosa pine]]
They are [[plant sexuality|dioecious]], 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 {{convert|50|mph|km/h}}, an example of [[rapid plant movement]].  The [[Arceuthobium americanum|lodgepole pine dwarf mistletoe,'' Arceuthobium americanum]], has been found to explosively-disperse its seeds through [[thermogenesis]].<ref name="Thermogenesis-triggered seed dispersal">[http://www.nature.com/ncomms/2015/150209/ncomms7262/full/ncomms7262.html], Rolena A.J. deBruyn, Mark Paetkau, Kelly A. Ross, David V. Godfrey & [[Cynthia Ross Friedman]]. Thermogenesis-triggered seed dispersal in dwarf mistletoe.</ref>  Dwarf mistletoe seeds are enveloped in a [[Hygroscopy|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, ''[[blue pine|Pinus wallichiana]]'' in the [[Himalayas]].

==Effects of parasitism==
In western forest ecosystems of [[North America]], 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 tissue]]s.  Dwarf mistletoes have a root-like [[Endophyte|endophytic]] system, composed of primary and secondary [[Haustorium|haustoria]], which invade, but do not injure, both the [[xylem]] and [[phloem]] of the host.  Because this root-like endophytic system is not [[soil]]borne, 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]] 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]].  The principal carbohydrate transported from the host to dwarf mistletoe is [[sucrose]].  Because dwarf mistletoes are phloem-deficient, they draw carbohydrates from their hosts by connections to the host phloem and ray [[parenchyma]].  The rate of carbohydrate transport varies by season, but dwarf mistletoes continuously draw carbohydrates from their hosts throughout the year.

==Dwarf mistletoe rating system==
<!-- [[Dwarf mistletoe rating system]] redirects here. -->
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 [[crown (botany)|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.<ref>{{cite web |title=Management guide for dwarf mistletoe |url=http://www.fs.usda.gov/Internet/FSE_DOCUMENTS/stelprdb5187427.pdf |access-date=25 July 2016 }}</ref>

== Species ==
[[File:Arceuthobium occidentale on Pinus sabiniana.jpg|thumb|''Arceuthobium occidentale'' shoots on ''Pinus sabiniana'' branches, Mount Diablo, California. 25 December 2008]]{{div col}}
* ''[[Arceuthobium abietinum]]'' <span class="Person">Engelm. ex Munz</span>
* ''[[Arceuthobium americanum]]'' <span class="Person">Nutt. ex Engelm.</span>
* ''[[Arceuthobium apachecum]]'' <span class="Person">Hawksworth & Wiens</span>
* ''[[Arceuthobium azoricum]]'' <span class="Person">Hawksw. & Wiens</span>
* ''[[Arceuthobium blumeri]]'' <span class="Person">A. Nels.</span>
* ''[[Arceuthobium californicum]]'' <span class="Person">Hawksworth & Wiens</span>
* ''[[Arceuthobium campylopodum]]'' <span class="Person">Engelm.</span>
* ''[[Arceuthobium cyanocarpum]]'' <span class="Person">(A. Nels. ex Rydb.) A. Nels.</span>
* ''[[Arceuthobium divaricatum]]'' <span class="Person">Engelm.</span>
* ''[[Arceuthobium douglasii]]'' <span class="Person">Engelm.</span>
* ''[[Arceuthobium gillii]]'' <span class="Person">Hawksworth & Wiens</span>
* ''[[Arceuthobium globosum]]'' <span class="Person">Hawksw. & Wiens</span>
* ''[[Arceuthobium laricis]]'' <span class="Person">(Piper) St. John</span>
* ''[[Arceuthobium littorum]]'' <span class="Person">Hawksworth, Wiens & Nickrent</span>
* ''[[Arceuthobium microcarpum]]'' <span class="Person">(Engelm.) Hawksworth & Wiens</span>
* ''[[Arceuthobium minutissimum]]'' <span class="Person">Hook. f.</span>
* ''[[Arceuthobium monticola]]'' <span class="Person">Hawksworth, Wiens & Nickrent</span>
* ''[[Arceuthobium occidentale]]'' <span class="Person">Engelm.</span>
* ''[[Arceuthobium oxycedri]]'' <span class="Person">(DC.) M.Bieb.</span>
* ''[[Arceuthobium pusillum]]'' <span class="Person">Peck</span>
* ''[[Arceuthobium siskiyouense]]'' <span class="Person">Hawksworth, Wiens & Nickrent</span>
* ''[[Arceuthobium tsugense]]'' <span class="Person">(Rosendahl) G.N. Jones</span>
* ''[[Arceuthobium vaginatum]]'' <span class="Person">(Humb. & Bonpl. ex Willd.) J. Presl</span> (Syn. ''Viscum vaginatum'' <span class="Person">Humb. & Bonpl. ex Willd.</span>)
{{div col end}}

==References==
{{Reflist}}

==Further reading==
* 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.

==External links==
* [http://www.parasiticplants.siu.edu/Viscaceae/images/CAM.male.JPEG Maleshoots of ''A. campylopodum'' in California]
* [http://www.parasiticplants.siu.edu/Viscaceae/images/TSU.shorepine.JPEG Fruiting plant of ''A. campylopodum'' on Pine shoot]
* [http://plants.usda.gov/java/profile?symbol=ARCEU PLANTS profile for Arceuthobium (United States) including range maps.]
* [http://www.fs.fed.us/rm/pubs/rmrs_gtr098/rmrs_gtr098_057_065 US forest service data about dwarf mistletoes]

{{Taxonbar|from=Q244255}}

[[Category:Arceuthobium| ]]
[[Category:Dioecious plants]]
[[Category:Epiphytes]]
[[Category:Parasitic plants]]
[[Category:Santalaceae genera]]