True Mistletoes

Please see Parasitic Plants for a general introduction to mistletoes in a broader context.

The genera Phoradendron and Viscum are called “true mistletoes” or “leafy mistletoes”, at least by forest pathologists, to distinguish them from the important dwarf mistletoes.

The original “mistletoe” is Viscum album, the European mistletoe. Worldwide, there are up to 130 species of Viscum distributed in much of the world outside the Americas. Phoradendron species are sometimes called American true mistletoes. There are roughly 240 species of Phoradendron, and they are restricted to the Americas.

True mistletoes are usually hemiparasitic. They take all their water and inorganic nutrients from their host but generally make most of their own organic food. They are green and most have recognizable leaves. Some species, especially those in desert habitats, have only small scales for leaves. Old stem infections of Phoradendron libocedri are known to stay alive for centuries in Calocedrus decurrens (incense-cedar) without producing external shoots ​[2]​, in which case they would certainly be holoparasitic.

Like all mistletoes, they are obligate parasites. They cannot develop without a host, and when the host dies, they die.

Vectoring is by birds. The fruits are usually one-seeded berries. Although some (like European mistletoe) have berries that are toxic to humans, birds disseminate seeds after eating the berries, either by rubbing off seeds stuck to the beak or by passing the seed in excrement. The seed usually goes through with its sticky coat intact, and if dropped on a young branch with thin bark on a suitable host, can then germinate and infect. In Phoradendron, cedar waxwings and euphonias are most important. Euphonia in particular is said to be adapted to Phoradendron. The mistletoe thrush is the most important vector of European mistletoe.

True mistletoes can cause drought stress. When water is limiting, mistletoes can maintain its supply through lower osmotic potential ​[2]​. As a tree is dying, there is often more mistletoe foliage than host foliage. Infected branches may be the last to die.

True mistletoes are generally not as damaging as dwarf mistletoes, but some can cause substantial growth loss when infections are numerous, some can kill portions of the tree, and as mentioned, can lead to mortality during drought. They are quite common and conspicuous in many areas.

Physiology

Mistletoes have some chlorophyll and engage in some photosynthesis, so they are not totally dependent on the host for everything. In the language of parasitic plants, this is known as hemiparasitism as opposed to holoparasitism, where the parasite gets all water, minerals, and fixed carbon from the host.

European mistletoe

The classic European mistletoe is Viscum album. It apparently has the widest host range of any mistletoe, including 452 taxa in 96 genera ​[1]​.

It was introduced to California about 1900 by Luther Burbank, the pioneering horticulturist, because he thought it was pretty (and it is; it has nice white berries and evergreen leaves, but looks quite like the native Phoradendron leucarpum in that area, so what was he thinking?) ​[6]​. He planted it on trees in his experimental farm in Sebastopol, north of San Francisco. In the early years it spread slowly, but moved faster in the late 20th century ​[7]​. It infects a wide variety of hardwoods there. As of 1991 it extended 12 km from the point of introduction and occupied a gross area of over 180 km². A survey in 2019 discovered that the maximum distance had doubled, to 24.6 km, and the gross area of infestation had risen to 606 km² ​[12]​. It is certainly too late to eradicate it.

In 1988, it was found about 1500 km north in Victoria, on Vancouver Island, British Columbia, Canada ​[4]​. It is thought to have been brought from Europe by a returning soldier in 1945 ​[3]​. In 1991 it was sparsely spread over about 114 km² ​[4]​. It apparently hasn’t spread much since then, and locations are kept secret to prevent eradication ​[3]​.

Folklore

European mistletoe is the stuff of legend! Mistletoe was thought to be sent to earth by the gods, using a bird that was considered to be a messenger of the gods, the mistletoe thrush. In fact, this bird migrates from Africa to Europe in late winter. From January to March, it feeds almost exclusively on mistletoe berries. Then it evacuates the seeds intact. It serves as an effective vector. The distribution and abundance of mistletoe is thought to be determined in part by the migration patterns of the mistletoe thrush.

Because it was sent by the gods, the plant was thought to have spiritual power. One aspect is medicinal. Through the middle ages and beyond, V. album has been used as medicine for all sorts of ailments. Amulets were worn to ward off illness. Leaves and teas were eaten. As pharmacology developed, research was done that suggested it could be useful in reducing high blood pressure and as a diuretic. Other benefits were claimed. Trials had mixed results though. Medicinal power was also believed in Japan, especially when the mistletoe grew on the sacred willow tree. Even Navajos used certain Arceuthobium species as medicines.

Several cultures seem to associate mistletoe with fertility. In both England and Japan, women were advised to eat mistletoe leaves to promote conception. In England, an old practice was to feed mistletoe shoots to the first cow to give birth each year; that would increase fruitfulness of the herd and protect their health. In Japan, a few mistletoe leaves were crumbled and sown with crop seeds to promote fertility. Southwestern Indians made a tea out of Phoradendron juniperinum in order to relax muscles for childbirth.

In parts of Europe, other sorts of spiritual power were associated with mistletoe, Viscum album. To the Druids, the oak tree represented God, and the mistletoe in the oak represented human dependence on God. In Germany, mistletoe could be brought into a haunted house. It was thought that ghosts and other evil spirits would be forced to come out of hiding and answer any questions the peasants had. Generally, it was thought to bring good fortune, so it was hung over doorways, especially in midwinter. It seems reasonable that people would hang an evergreen plant indoors in winter just for the summery feeling it gives. The power is just a bonus. We don’t know how the custom of kissing under mistletoe originated, but one connection is that Vikings associated mistletoe with their goddess of love, Frigga. It makes sense that if mistletoe is hanging up around Christmas time and has the power of good fortune and fertility, that getting two lovers together would be evidence of its power. Supposedly a man should remove one berry from the plant for each kiss he steals from the woman beneath it. When the berries are gone, so is the opportunity for kissing!

In America, the mystical power and symbolism has been vested in Phoradendron leucarpum. This is sometimes sold as Christmas mistletoe; most of it collected in Texas.

Phoradendron (American True or Leafy Mistletoes)

• American true or leafy mistletoe.
• Most are leafy, some are leafless in desert regions (technically there are minute, scale-like leaves).
• They tend to have longer, stouter, more persistent shoots than dwarf mistletoes.
• Hosts are hardwoods mostly, but some conifers in the west.
• Occur in warm-temperate to tropical Americas.
• Like most Viscum spp., Phoradendron spp. are dioecious (plants have only male or only female flowers).

The species of Phoradendron have gone through stages of lumping and splitting, so names have changed over the years. Here we follow the treatment of Nickrent in Flora of North America ​[10]​, which is quite different from the traditional arrangement. Some of it is a little hard to swallow for a pathologist, because we like to see pathogen species have some relationship to hosts. Some important or widespread species in the United States are:

On Hardwoods

Phoradendron leucarpum (oak or American mistletoe). This species is an amalgam of other species that have been synonymized because genetic and morphological variation does not support them ​[10]​. It includes the old eastern mistletoe, P. flavescens, as well as P. tomentosum in the central south from New Mexico to Mississippi, and the western species P. macrophyllum (bigleaf mistletoe). It occurs from New Jersey and Pennsylvania south, from Oregon south, across the southern states and into Mexico. It is the only true mistletoe in eastern USA (except for a Caribbean species that occurs in North America only on Key Largo, Florida). It has a very wide host range, but in a local area it is usually confined to one or several host species, suggesting locally adapted, host-specialized populations. This species resembles Viscum album and is collected in Texas and sold as Christmas mistletoe.

P. villosum (Pacific, hairy, or western oak mistletoe). This western species occurs abundantly from Oregon to California and Mexico, but extends sparsely east across Arizona and New Mexico to western Texas. Although it can cause large growths on Quercus spp. ​[2]​, it was found to cause no growth loss or mortality in one study ​[8]​. However, in Oregon it is said to cause slow growth, predispose trees to insect attack, and lead to mortality during drought ​[11]​. Two subspecies are recognized, P. v. subsp. villosum in Oregon, California and Baja California, and P. v. subsp. coryae from Arizona to Texas and northern Mexico east of Baja California. (see photos near top)

P. californicum (desert mistletoe). This leafless species occurs in desert habitats in southern California, Nevada, Arizona, and Mexico. It grows chiefly on leguminous species such as Parkinsonia spp. (palo verde) and Prosopis spp. (including mesquites).

On Conifers

P. bolleanum. This is a mashup of three major taxa formerly considered as species. They are:

• P. bolleanum sensu stricto (in the strict sense). This variant occurs from Texas to central Mexico, parasitizing Juniperus spp. but also Arbutus spp. (which are not conifers).
• P. pauciflorum (fir mistletoe). This one ranges from California east to western Arizona and south into Baja California. It is unusual in being host-specific, only attacking Abies concolor (white fir), although it was found rarely on Cupressus arizonica in one location in Baja Calfornia ​[5]​. It tends to occur high in the tree and commonly kills tops above the point of a stem infection. Fun fact: Abies concolor is the only tree species in the USA parasitized by both Phoradendron and Arceuthobium.
• P. densum (dense mistletoe). Oregon, California, Arizona, and into Baja California. This variant infects both Cupressus (cypress) and Juniperus spp. It is named for its compact habit.

P. juniperinum. This leafless species includes what was traditionally recognized as P. juniperinum, but also the incense-cedar pathogen (considered subspecies by some authors):

• P. juniperinum sensu stricto (juniper mistletoe). This is found in Mexico and the Southwest, including west Texas and up to Colorado, Utah, Nevada, Oregon. It attacks Juniperus spp. and rarely a few other species in certain locations ​[5]​. Most junipers grow at low elevations in relatively hot, dry climates. Perhaps for that reason, like desert mistletoe, this species is leafless. However, in Viscum, it has been suggested that leaflessness is associated with higher parasitic specialization ​[9]​. (see photos and featured image at top, close-up of a female plant with fruit and pairs of minute, pointed, scale-like leaves at nodes).
• P. libocedri (incense-cedar mistletoe). This is host-specific if recognized as a species, restricted to Calocedrus decurrens (incense-cedar). It occurs throughout the range of its host, in Oregon, California, western Nevada, and Baja California. Unlike most Phoradendron spp., the shoots are pendant, hanging from the point of attachment. It causes severe growth suppression when abundant. Stem infections often produce no shoots and can remain alive inside the stem for hundreds of years ​[2]​.