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 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 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 abietinum Engelm. ex Munz Arceuthobium americanum Nutt. ex Engelm. Arceuthobium apachecum Hawksworth & Wiens Arceuthobium azoricum Hawksw. & 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 Hook. f. Arceuthobium monticola