The Lady Slipper newsletter of the Kentucky Native Plant Society has been published since the Society’s founding in 1986. We occasionally feature an article from a past issue. This one, about the parasitic plants known as “mistletoes”, which first appeared in the winter of 2004, Vol. 19, No. 4, seemed appropriate for the season. If you would like to see other past issues, visit the Lady Slipper Archives, where all issues from Vol. 1, No. 1, February 1986 to Vol. 34, No. 1, Winter/Spring 2019 (after which we moved to this blog format) can be found.
The Mistletoes
by David Taylor, US Forest Service
As autumn fades into winter, dark green clumps perched in trees along roads and fencerows, stand out once again. Some trees, especially black cherry, now exhibit gnarled or stub branches, the telltale sign of a current or past infestation, even if the plant is unseen. Birds may be seen plucking white berries from the clumps, and occasionally a person may be seen scouting trees from which to gather some at a later date. The object of attention?— mistletoe.
In Kentucky and neighboring states, we think of the thick-leaved, usually dark green plant we see growing in hardwood trees in towns and along country roads. This is only one of many species of plants known as mistletoe. Before returning to the mistletoe familiar to us, we will take a survey of the mistletoes.
Mistletoe Families
The plants commonly known as mistletoes belong to one of two families, the Loranthaceae and the Viscaceae. Two other less familiar families of ‘mistletoes’ are the Eremolepidaceae and the Misodendraceae. All have in common a hemiparasitic relationship with a host plant, almost always a woody plant. Hemiparasites derive water, minerals, and occasionally food (sugars) from the host plant, but are photosynthetic and produce at least part of their own food. The lesser known families are briefly mentioned first.
The Eremolepidaceae family is variously placed in the Loranthaceae or the Santalaceae depending on the author (see Watson and Dallwitz 1992–2003, Nickrent, 2003a). It is a small family of 4 genera with 12 species. They are distributed in New World tropics. All are small shrubs, with attachments made to stems of woody plants. They may have well developed leaves or scale-like leaves. Flowers are somewhat showy or small and inconspicuous. Seeds form in berries in which they are coated with viscous material. Seeds are largely bird dispersed. They are found the West Indies (Antidaphne, Eubrachion, Eremolepis— =Antidaphne in some views, see W 3 Tropicos 2003) and South America (Antidaphne, Eubrachion, Lepidoceras).
The Misodendraceae is a small family of one genus (Myzodendron) and 11 species They are distributed in the New World tropics, southwest South America, and Antarctica. All are small shrubs with attachments made to stems of Notofagus, southern beech (Nickrent 2003). Leaves are either scale leaves or well developed. Flowers are mostly small and inconspicuous. The fruit is dry and seeds are wind-dispersed.
The Loranthaceae was once considered a large diverse family, but in the last 40 years, a subset of the family has been recognized as the Viscaceae. The Loranthaceae in the strict sense is a tropical family inhabiting both the Old and New World tropics, but with species found in temperate regions. About 70 genera and over 900 species are known in the family ranging in habit from trees (Nuytsia) to shrubs to lianas (Watson and Dallwitz 1992–2003). Most genera in the family and all of the mistletoes make their parasitic attachments to stems and branches of woody plants, but a few genera attach to roots. The plants are generally evergreen and have either normal or scale leaves. Flowers tend to be large and showy. Fruits are usually berries and most seeds are covered with a viscous coating. Seeds are primarily bird-dispersed. North (Central) American genera include Cladocolea, Struthanthus, Psittacanthus, and Dendropemon
Cladocolea is a poorly understood genus consisting of about 23 mostly rare species, predominately found in central and southern Mexico. These species occur as shrubs or nearly vines in both hardwoods and conifers.
Struthanthus consists of 50–60 species occurring from northern Mexico (very close to the U.S.) to southern South America. This too is a difficult group of species and many are rare. They occur primarily in hardwoods, but also infect conifers.
Psittacanthus (parrot flower) is found throughout the New World tropics, as far north as central Mexico (Geils, et al. 2002). Most members of this genus have large, showy—often red or orange—flowers (see a photograph at Nickrent 2003).
Dendropemon occurs further south in Central America and South America. Dendropemon is also known from the West Indies with some species endemic to the region (see Correll and Correll 1982). The species in these latter two genera tend to be rare, found in small scattered patches. Individual plants of these genera are usually vine-like or small shrubs. They usually occur in hardwoods but are sometimes found in conifers and sometimes in enough concentration to cause economic damage to timberlands.
The Viscaceae is the family of ‘true’ mistletoes. The family consists of 7 genera and about 450 species. The family is dispersed in temperate and tropical climates of both the Old and New Worlds, but is best developed in the tropics. All species are shrubs with attachments to branches and stems of woody plants. Leaves are leathery to herbaceous or scale-like. Flowers tend to be small and inconspicuous. Seeds are produced in a berry filled with viscous tissue (Watson and Dallwitz 1992– 2003). Seeds are primarily bird-dispersed. North American genera are Arceuthobium (also occurs in Europe and Asia) and Phoradendron. Viscum occurs in Europe and has been introduced into California and Canada (Geils et al. 2002).
Phoradendron (“tree thief”) consists of about 200 species scattered through the temperate and tropical regions of the New World. Most of its distribution is in Mexico and Central America. About 12 species are found in the U.S. and one, Phoradendron leucarpum (also seen in recent literature as P. serotinum and in older literature as P. flavescens), in the southeast U.S. including Kentucky. Species of Phoradendron tend to be dark green, although ailing and dying plants may be light green, and a few are reddish. Most have typical, but leathery leaves. A few have reduced leaves. Plants are either monoecious or dioecious. They produce small inconspicuous flowers and a small white, pink or red berry, which contains 1–2 seeds and a sticky material, viscin, by which the seeds stick to tree limbs. Birds are known to spread the seeds by wiping beaks and feet clean of the viscin and attached seed on limbs. Gravity also disperses seeds, usually with the same tree. Individual plants may reach more than a meter across, but usually are about 1/3 meter across.
The genus is most frequent in hardwood trees, especially (in the U.S.) oaks, maples, black walnut, hackberries, black cherry, mesquites, and manzanitas. The common mistletoe of Kentucky is in fact sometimes called oak mistletoe because of a tendency to occur on oaks especially in the southeast U.S. Some species are conifer specialists infecting primarily junipers, but also true cedars, firs and pines. A few species will infect both hardwoods and conifers. A few are also known to infect…mistletoes (Geils et al. 2002). This double layer of parasitism is known as epiparasitism or hyperparsitism. In the Southeast and Kentucky, infections are in hardwood trees. R.L. Thompson and coworkers (Thompson 1992, Thompson and Noe 2003) of Berea College have looked at the distribution of mistletoe in host trees in Kentucky. T.E. Hemmerly and students (Rucker and Hemmerly 1976, Hemmerly et al. 1979, Hemmerly 1981, Hemmerly 1989) of Middle Tennessee State University have looked at the distribution and host specificity of mistletoe in Tennessee.
Phoradendron seldom causes economic damage to forest lands, but has been documented in nut tree and fruit orchards where the loss can be substantial. In some cases, infestations of a magnitude to cause significant mortality occur, usually in conifers. Individual trees anywhere may succumb to vigorous infestations of Phoradendron and treatments, although difficult, are available (Nickrent 2003, Geils et al. 2002), but the mistletoe does not seriously affect most trees.
Arceuthobium (“juniper life,” from its parasitic nature on junipers and related trees), the dwarf mistletoes, consist of about 42 species distributed in both the Old and New Worlds. About 39 of these are restricted to North and Central America. The plants range in color from green to orange to reddish or even black. They have greatly reduced scale leaves, leaving the stems as the major photosynthetic organs. They produce small inconspicuous flowers and plants are dioecious. The fruit is a 1- seeded, sticky berry, which in most species is hydrostatically explosive. Spread is through this explosive action. Individual plants are seldom over 20 cm across, and often as small as a few centimeters across. Some workers have submerged several of the species into Phoradendron (USDA-NRCS 2003). The genus is a conifer specialist infecting members of the Pinaceae (pines, firs, Douglas fir, hemlock, spruce) and Cupressaceae (junipers, true cedars). In the U.S. (Rockies and west), pines and junipers are frequently infested. The genus does not occur in the southeast U.S., but one species, A. pusillum, occurs in the Great Lakes states and the Northeast.
Arceuthobium frequently causes economic damage to forest lands. Most damage occurs in pine forests, but damage also occurs in Douglas fir forests. I have seen mixed white and yellow pine forest at 3000 m elevation in the Sierra Madre Occidental of Mexico covered with Arceuthobium plants. Studies have shown that about 60 percent of the explosively ejected seeds end up on the original host plant resulting in additional infection. About 90 percent of the remaining 40 percent lands on adjacent trees (Geils et al. 2002). The net result is rapid, efficient spread of the mistletoe that results in both decreased growth and death of trees. Numerous control strategies exist (see Geils et al. 2002). The extent to which this genus can become a pest is reflected in the formation of a research group to study it, The Mistletoe Center at the Rocky Mountain Research Station (see https://www.fs.usda.gov/research/treesearch/4812 ) and publications such as that by Geils et al. (2002). Viscum (“sticky,” in reference to the coating around the seed) is an Old World genus of about 150 species occurring in both temperate and tropical regions. It infests primarily hardwood trees, especially oaks. The common European mistletoe, V. album, also infects apples and some evidence points to a decline in this species as apple orchards are converted to other uses (Briggs 2002). It is generally yellow green with normal, somewhat leathery leaves. It produces small inconspicuous flowers and a 1–2 seed berry within a viscous substance. Seeds are bird dispersed in the same way that those of Phoradendron are. It seldom causes economic damage to forest lands.
The Lore of Mistletoes
Mistletoes figure in legends, customs and medicines. Aeneas, the father of the Roman people according to Virgil, sought the ‘golden bough’ when he chose to visit hell. This golden bough, Viscum album, provided him with protection and magical powers on his journey. Whether the Druids of the British Isles were aware of this myth is not known, but they revered the plant as well.
The mistletoe of the Druids is Viscum album. It is not certain why this plant held the prominence in Druidism it did. Suggestions have been made, however. Tainter (2002) provides an easily read summary. To the Druids, oaks were sacred. It is surmised that a plant appearing live in winter, which also grew on some of the sacred oaks was viewed as more sacred or magical than even the oaks (although much is speculation—see Briggs 2002). Myth or legend has it that mistletoe was used in some sacrificial rituals. Conversion of the Celts to Christianity diminished the practices of the Druids, but some belief in the magical powers of mistletoe helped to propel the plant into the Christmas season custom of kissing under the mistletoe. Briggs (2002) suggests this custom arose from the mistletoe’s symbol of friendship rather than of fertility. The custom was brought to the New World with English colonists who found Phoradendron leucarpum along the mid-Atlantic coast and in southeast colonies. New Englanders are known to have used the small, scale-leaved Arceuthobium pusillum (eastern dwarf mistletoe) in much the same way.
Mistletoes in Medicine
Like many plants, mistletoes have been used for medicinal purposes, and like many plants, the characteristics that make them useful as medicines also make them toxic. Phoradendron mistletoes are harvested as nutritional forage for livestock in some areas of Central and South America (Geils et al. 2002). Cattle poisoning has been reported (HerbMed, 1998–2003). People in these areas consume tinctures of Phoradendron species as a stimulant tonic and medicines for childbirth (stopping post-partum hemorrhage) and other conditions (Geils et al.). A number of recent trials have looked at Phoradendron and Viscum extracts for treating (slowing) some cancers and HIV (HerbMed 1998–2003). In recent years, health food stores sold mistletoe ‘teas.’ These mistletoes, however, contain toxic amines (tyramine and beta-phenylethylamine). Both are known to cause gastroenteritis and an occasionally fatal drastic lowering of blood pressure (Blackwell 1990, Tampion 1977). Some individuals develop dermatitis after contact with the plant. Other studies have shown that even with the ingestion of a few berries or leaves, toxicity is mild, but may include seizures (HerbMed 1998–2003). Viscum is believed to have similar properties (Tampion 1977). At one time, doctors in France widely prescribed mistletoe to lower blood pressure.
Mistletoes at Home and Beyond
As you drive around Kentucky this winter, especially central Kentucky, look for the telltale balls of mistletoe in hardwood trees. Remember that is just one of many species of Phoradendron mistletoes and one of about 1500 species of mistletoes worldwide. Consider the human history of mistletoes presented above and search your library and the web for more.
References
Blackwell, W.H. 1990. Poisonous and medicinal plants. Prentice Hall, Englewood, NJ. 329 p.
Briggs, J. 2002. Mistletoes. Website. Accessed 10 November 2003 at https://www.mistletoe.org.uk/
Correll, D.S. and H.B. Correll. 1982. Flora of the Bahama Archipelago. J.Cramer, Hirschberg, Germany. 1692 p.
Geils, B.W., J. Cibrian Tovar, B.Moody, tech. coords. 2002. Mistletoes of North American conifers. Gen. Tech. Rep. RMRS-GTR-98. U.S. Department of Agriculture, Forest Service, Rocky Mountain Research Station, Ogden, UT. 123 p.
Hemmerly, T.E. 1981. Host specificity of mistletoe in middle Tennessee V: Williamson County. J. Tennessee Acad. Sci. 56:77–78.
Hemmerly, T.E. 1989. Mistletoe parasitism in Tennessee. J. Tennessee Acad. Sci. 64:121–122.
Hemmerly, T.E., A.A. Forsythe, and M.L. Womack. 1979. Blackgum—exclusive host of mistletoe in Lawerence County, Tennessee? J. Tennessee Acad. Sci. 54:89–90.
HerbMed. 1998–2003. Web application: an interactive, electronic herbal database. Alternative Medicine Foundation. Accessed 13 November 2003 at https://www.herbmed.org/
Nickrent, D. 2003a. The parasitic plant connection. Website. Accessed on 18 October 2003 at https://parasiticplants.siu.edu/
Rucker, E. and T.E. Hemmerly. 1976. Host specificity of mistletoe in middle Tennessee I: Rutherford County. Castanea 41:31–33.
Tainter, F.H. 2002. What does mistletoe have to do with Christmas? Feature story December 2002, APSnet. American Phytopathological Society, Minneapolis, MN. Available at https://www.apsnet.org/edcenter/apsnetfeatures/pages/mistletoe.aspx
Tampion, J. 1977. Dangerous plants. Universe Books, New York, NY. 176 p.
Thompson, R.L. 1992. Host occurrence of Phoradendron leucarpum in the Lexington-Blue Grass Army Depot, Blue Grass Facility, Madison County, Kentucky. Trans. Kentucky Acad. Sci. 53:170–171.
Thompson, R.L. and F.D. Noe, Jr. 2003. American mistletoe (Phoradendron leucarpum, Viscaeae) in Rockcastle County, Kentucky.J. Ky. Acad. Sci. 64:29–35.
USDA-NRCS. 2003. The PLANTS Database. Web Application. National Plant Data Center, Baton Rouge, LA 70874-4490 USA. Accessed 13 November 2003 at https://plants.sc.egov.usda.gov/home
W3 Tropicos. 2003. Web Application: provides access to the Missouri Botanical Garden’s VAST (VAScular Tropicos) nomenclatural database and associated authority files. Missouri Botanical Garden, St. Louis, MO. Accessed 10 November 2003 at https://www.tropicos.org/home
Watson L. and M. J. Dallwitz. 1992–2000. The families of flowering plants: descriptions, illustrations, identification, and information retrieval. Version: 14 December 2000. Accessed 19 October 2003
David Taylor, Forest Botanist for the Daniel Boone National Forest for the last 34 years, works with rare plants, invasive plants and plants in general. Most of his hiking and plant observations occur in the eastern and east-central parts of Kentucky. He encourages native plants and works to remove invasive plants on his property in central Kentucky.