Editing Arceuthobium (section)

==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.