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Phoradendron is a genus of mistletoes, native to warm temperate and tropical regions of the Americas. 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 10–80 cm (3.9–31 in) long, which grow on other trees. The foliage is dichotomously branching, with opposite pairs of leaves; these are fairly large, 2–5 cm (0.79–2.0 in) 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 of 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. Different species of Phoradendron tend to use different host species, though most species are able to utilise several different hosts.
The flowers are inconspicuous, greenish-yellow, 1–3 mm (0.039–0.12 in) diameter. The fruit is a berry, white, yellow, orange, or red when mature, containing several seeds embedded in very sticky juice; the seeds are dispersed when birds (notably Cedar Waxwings and Phainopeplas) eat the fruit, and remove the sticky seeds from the bill by wiping them on tree branches where they can germinate.
The foliage and berries of some species are toxic. Leafy mistletoe seldom kill but they cause stress during and reducing crop productions in fruits and nut trees.
Host and Symptoms
Phoradendron species can infect many varieties of plants including Hackberry (Celtis spp.) mesquite, cedar, elm and Osage-orange. Certain species of Phoradendron infect specific plants; for example, in Arizona, Phoradendron tomentosum infects cottonwood (Populus fremontii), sycamore (Platanus wrightii), ash (Fraxinus spp.), walnut (Juglans spp.) and willow (Salix spp.). P. californicum infects shrubs and trees such as acacia (Acacia spp.) and blue verde (Cercidium floridum). Other Phoradendron species of the mistletoe also infect junipers (Juniperus spp.) and oak (Quercus spp.). Branches often become swollen and distorted, forming burls and making the tree more susceptible to insect attack. Phoradendron presents serious problems along rivers, streams, parks, and golf courses with large cottonwood trees, where mistletoe growing from the branches of an infected tree can be seen. Deciduous trees can be mistaken for evergreens during the winter if infection is heavy. Other common symptoms include swelling formations of witch's broom, dieback, and weakened branches.
Phoradendron is a hemiparasite, meaning that it produces its own chlorophyll, but relies on the host plant to provide water and essential elements for growth and survival. Birds are the primary means of dispersal of the plant parasite. Birds consume the seeds inside of the fruits of leafy mistletoe, also called drupes, and excrete or regurgitate the seeds onto the branches of which the birds perch. 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 the haustorium extracts water and minerals, primarily carbon and nitrogen compounds, from the host tree or plant. The most important birds for effective dispersal include cedar waxwings, euphonias, silky flycatcher, bluebirds, thrushes, robins, and solitaires.
Leafy Mistletoe parasitizes a broad range of trees common in amenity and natural landscapes in the United States and the Americas. Commonly infected trees include cherry, walnut, beech, and other tree species. Although some mistletoe species show host specialization, new sites and new host species have been reported for Phoradendron. As stated earlier, Phoradendron is native to warm temperate and tropical regions of the Americas where winter temperatures are consistently warmer. Phoradendron species primarily infect broad-leaved tree species such as hackberry, mesquite oak, and elm in USDA Zone 6 (a hardiness zone in which inhabiting plants can withstand minimum temperatures of −10 °F) and warmer in the United States..As are all plants, Phoradendron is subject to death at extremely low temperatures.
Leafy mistletoe can adversely affect trees growing in urban environments and in forests. Although mistletoe is photosynthetic, it is an obligate, semi-parasitic evergreen plant that infects host plants to derive support, water, and essential elements. They are considered a nuisance in urban environments because of their appearance on deciduous trees during winter. Severe colonization of mistletoe in individual trees can adversely affect their health. Add mistletoe to a plant stressed by urban living and the result may be death for the host plant.
Control operations include affected watering trees to improve vigor and tolerance of the infestation, sanitizing the environment by removing infected trees, branches, and aerial shoots. Pruning infected branches is not generally sufficient, the hastoria are throughout the tree by the time infestation is noted. If desired, aerial shoots can be knocked off but generally the mistletoe quickly resprouts, some benefit is derived from annual removal because it reduces seed production and spread of mistletoe to nearby plants.
Removing the shoots does not eliminate the mistletoe infection but does reduce damage resulting from its reproduction. Covering infected branches with tarpaper, roofing tar or creosote has not proven to be effective, nor is it an attractive method. These treatments work by prohibiting photosynthesis of the haustorium by opaque coverings preventing light absorption. Perhaps the best way for discouraging additional bird-dispersal of mistletoe seeds is with branch pruning or shoot removal, since it is often the mistletoe fruit that initially attracts the birds. Given the modest damage and slow rate of increase of these mistletoes, these methods are usually sufficient.
After seeds germinate, they produce haustoria that penetrate the host to extract water and minerals. This endophytic portion results in a challenging control problem because treatments must kill the outer ectophytic portion of the plant as well as the endophytic portion without damaging the host.
Wildlife & Human Uses
Phoradendron mistletoes that infect conifers are widely distributed in the Western United States and Mexico on a number of common and valuable hosts. In the United States, Phoradendron are most important in California on incense cedar and true fir across the Southwest from California to Texas on junipers.
Extensive infections and mortality are uncommon in ashes. However, infections occur typically in open-grown trees (grown in a scattered arrangement in the plant community). These mistletoes are used as greenery in Christmas decorations.
Many species of hardwood trees are affected by mistletoe, but oaks and hickories are most commonly attacked. The impact of infestation is not normally severe, but the parasite may lower individual branch vigor. Where infestations are severe, tree decline may progress to the point where insect and fungus pests combine to kill trees.
- Phoradendron aequatoris Urb.
- Phoradendron anceps (Spreng.) G.Maza – Goldenfruit Mistletoe
- Phoradendron argentinum
- Phoradendron barahonae Urb. & Trel. – Island Mistletoe
- Phoradendron bolleanum (Seem.) Eichl. – Bollean Mistletoe
- Phoradendron californicum Nutt. – desert mistletoe, mesquite mistletoe
- Phoradendron canzacotoi Trel.
- Phoradendron capitellatum Torr. ex Trel. – Downy mistletoe
- Phoradendron coryae Trel. – Cory's Mistletoe
- Phoradendron crassifolium
- Phoradendron densum Torr. ex Trel. – Dense Mistletoe
- Phoradendron dichotomum (Bertero) Krug & Urb. – Bertero's Mistletoe
- Phoradendron emarginatum
- Phoradendron flavescens
- Phoradendron hawksworthii (DC.) Griseb. – Hawksworth's Mistletoe
- Phoradendron hexastichum (DC.) Griseb. – Tropical Mistletoe
- Phoradendron hieronymi
- Phoradendron juniperinum A.Gray – Juniper Mistletoe
- Phoradendron leucarpum (Raf.) Reveal & M.C.Johnst. (syn. P. flavescens, P. serotinum, P. tomentosum) – Oak (or Eastern) Mistletoe
- Phoradendron libocedri (Engelm.) Howell – Incense-cedar Mistletoe
- Phoradendron liga
- Phoradendron macrophyllum (Engelm.) Cockerell – Colorado Desert Mistletoe
- Phoradendron madisonii Kuijt
- Phoradendron mucronatum (DC.) Krug & Urban – Needletip Mistletoe
- Phoradendron pauciflorum Torr. – Fir Mistletoe
- Phoradendron piperoides (Kunth) Trel. – Piper Mistletoe
- Phoradendron pomasquianum Trel.
- Phoradendron quadrangulare (Kunth) Griseb.
- Phoradendron racemosum (Aubl.) Krug & Urb. – Bigleaf Mistletoe
- Phoradendron rubrum (L.) Griseb. – Mahogany Mistletoe
- Phoradendron tetrapterum Krug & Urb. – Fourpart Mistletoe
- Phoradendron tomentosum (Lam.) Griseb. – leafy Mistletoe
- Phoradendron trinervium (Lam.) Griseb. – Angled Mistletoe
- Phoradendron tucumanense
- Phoradendron villosum (Nutt.) Nutt. – Pacific Mistletoe
- Phoradendron wiensii Kuijt
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