Phoradendron

Genus of mistletoes
Automatic taxobox
| image = Phoradendroncalifornicum2. jpg
| image caption = Phoradendron californicum fruit
| taxon = Phoradendron
| authority = Nutt. Phoradendron is the largest genus of mistletoe in the Americas, and possibly the largest genus of mistletoes in the world. Traditionally, the genus has been placed in the family Viscaceae, but recent genetic research acknowledged by the Angiosperm Phylogeny Group shows this family to be correctly placed within a larger circumscription of the sandalwood family, Santalaceae. They are woody hemi parasitic shrubs with branches long, which grow on other trees. The foliage is dichotomously branching, with opposite pairs of leaves; these are fairly large, long, green and photosynthetic in some species (e. g. P. leucarpum), but minimal in some others (e. g. P. californicum). Although they are able to photosynthesize the plant relies on its host for some nutrients. The plant draws its mineral and water needs, and some of its energy needs, from the host tree using a haustorium which grows into the stems of the host. The flowers are inconspicuous and incomplete, no petals and 3 4 greenish yellow sepals, diameter. The fruit is a berry, white, yellow, orange, or red when mature, containing one to several seeds embedded in very sticky juice, called viscin. Phoradendron plants can be distinguished from mistletoes in other genera in Viscaceae by their inflorescences, which lack leaves and come from a single branching point or apical meristem. Certain species of Phoradendron are host specific; for example, in Arizona, Phoradendron tomentosum infests cottonwood (Populus fremontii), sycamore (Platanus wrightii), ash (Fraxinus spp. ), walnut (Juglans spp.) and willow (Salix spp.). P. californicum infests shrubs and trees such as acacia (Acacia spp.) and blue palo verde (Parkinsonia florida). Some species infest junipers (Juniperus spp.) and oaks (Quercus spp.). Branches often become swollen and distorted, forming burls and making the tree more susceptible to insect attack.

Phoradendron species are hemiparasites which produce their own chlorophyll but rely on the host plant to provide water, minerals, and other nutrients. Birds are the primary means of dispersal of the parasite. Birds consume the drupes of the mistletoe and excrete or regurgitate the seeds onto the branches of the host plant. The seeds do not need to be ingested to germinate. Germinating seeds produce a radicle, a holdfast, and eventually the germinated seeds produce haustoria. The haustorium is a root like structure that penetrates the host plant's bark and cambium, reaching the xylem and phloem where it extracts water and minerals, primarily carbon and nitrogen compounds. A study on the nutrient ratio between Phoradendron and their hosts found that the parasite have higher concentrations of nitrogen and minerals, especially in leguminous hosts. This suggests that the parasite actively draws nutrients from the host plant through both the xylem and phloem, challenging the alternative theory of the passive uptake of nutrients by the parasite from the host xylem only. The most important birds for effective dispersal include the cedar waxwing, euphonias, silky flycatchers, bluebirds, and thrushes. Control and management regimes include watering the host plant to improve its vigor, and removing infested vegetation. Pruning infested branches is not generally effective because the haustoria can infiltrate deeply. Plucking the mistletoe herbage is a temporary treatment because it easily resprouts, but keeping its herbage sparse can help to reduce its seed production. Phoradendron are the preferred food of the Phainopepla, a silky flycatcher. The male defends territories where fruiting mistletoe is abundant. There may actually be some mutualistic interactions between the parasite and the host in some Phoradendron species. The presence of Phoradendron juniperinum on host Juniperus monosperma, for example, has been suggested to increase dispersal of the host's seeds by birds. The mistletoe berries may attract frugivorous birds (e. g. thrushes) to eat the host juniper's seeds and disperse them. The net benefit on the hosts is difficult to quantify, however. The birds dispersing the host's seeds and benefiting the host, while at the same time dispersing the mistletoe's seeds and spreading the Phoradendron infection. Phoradendron californicum was used by Native American peoples as a food and medicinal remedy. Some Phoradendron species, such as Phoradendron serotinum and Phoradendron flavescens, are used in North America as Christmas decoration, substituting for the European mistletoe Viscum album. They are harvested commercially for that purpose.