extensive damage. But unlike most Mistletoe: Its Role in plant pathogens, they seldom kill the host. Thus, these parasites seem be- nign compared with other disease-caus- Horticulture and Human ing organisms, and the leafy mistletoes with which most of us are familiar are more likely to be considered an oddity Life or at most a disfigurement than a plant pathogen. Laura K. Paine1 and Helen C. Harrison2 Mistletoe taxonomy Classification of the mistletoes has Additional index words. Viscum album, Mistletoe and other plants that been an ongoing process for centuries, Phoradendron spp. remain green in winter have long been beginning perhaps in the writings of associatedwith winter solstice and New Theophrastis and Pliny, who noted at Summary. Parasitic flowering plants Year celebrations and were gradually least three different groups or species represent a unique ecological adapta- incorporated into the Christian tradi- in their descriptions (Kanner, 1939). tion, having evolved away from inde- tions surrounding the birth of Jesus Until quite recently the mistletoes were pendent function and toward an in- (Kanner, 1939). Few who partake of grouped into one large family, the creasing dependence on other higher the kiss beneath the mistletoe bough Loranthaceae, containing two subfam- plants for survival. Mistletoe, a com- on Christmas Eve are aware of the mon evergreen parasite of woody ilies, the Loranthoideae and the Vis- plants, has played a significant role in long-term cultural significance of this coideae. human culture for centuries. plant or that these festive sprigs of Current thinking places the two Throughout history, mistletoe species green leaves and tiny white berries sur- groups in separate families, based on were nurtured and revered as medici- vive in life as a parasite on other plants. flower morphology. The Loranthaceae nal herbs and religious symbols. But Characteristics of parasitic produce comparatively large, showy the role of mistletoe has changed. Its flowers, while those of the Viscaceae importance in western culture has plants tend to be small and inconspicuous. dwindled to a minor, though endur- There are several families of seed- Other, less noticeable, differences in ing, association with the Christmas bearing plants that parasitize other floral structure support this division holiday. In contrast, its significance as a parasite of tree crops and woody plants for some or all of their needs. (Calder, 1983). Both families occur ornamentals has increased in recent Within this group, there are several mainly in the tropics, only a few species years. Mistletoe species are studied in unrelated families ranging from those, being found in temperate regions. The efforts to control their pathogenic such as Spanish moss, that merely de- Viscaceae, of which the traditional effects and to gain insight into the pend on the host for support, to those, Christmas mistletoe is a member, is evolutionary role played by this family like the dodders, that contain no chlo- the smaller of the two families, con- of parasitic flowering plants. The rophyll and depend on the host for all taining less than half the number of unique characteristics of mistletoe their requirements. The mistletoes fall species of the Loranthaceae. that challenge horticultural research- between these two extremes and are There are seven genera of the Vis- ers have contributed to its enduring classed as “hemi-parasites”; that is, caceae, the largest being Phoradendron, role in human life. they produce some or all of their own a New World genus of »170 species, energy through photosynthesis but de- whose center of diversity is tropical pend on the host for water and minerals. South America. A close relative, Den- The cultural significance of In general, mistletoes are para- drophthora, centered in the Caribbean, sites of the aerial portions of woody includes 55 species. The genus Ar- plants plants, although root parasitism is ceuthobium is the only group with Because human subsistence is so known among them (Calder, 1983). species native to both the Old and closely tied to the plant kingdom, it is Most mistletoes are perennial, devel- New Worlds, although it is commonly not surprising that, in many cultures, oping woody stems and an evergreen considered a New World genus; 24 of some plant species have taken on sig- habit, even in temperate regions. In its 29 species occur in North America. nificance beyond their utilitarian role many species, flowering and seed pro- Viscum is also a widespread genus, and have become symbols associated duction occur only after several years with 100 species ranging across Africa, with rituals, holidays, and other occa- of growth and, once established, a Madagascar, Europe, Asia, Malaysia, sions. Mistletoe is such a plant, and it mistletoe will often survive for a de- and eastern Australia. Other genera has a long history of association with cade or more (Scharpf and Hawks- include Korthalsella, with more than seasonal rituals in a number ofcultures worth, 1974). All mistletoes cause some 30 species, Notothixos with eight spe- (Gill and Hawksworth, 1961; Kanner, degree of disease response in their cies, and Ginalloa with five species. 1939). host; that is, they cause disruption of The latter three are tropical (Barlow, 1 the normal physiological and biochem- 1983). Graduate Research Assistant. 2 ical functioning of the plants that they The genera discussed herein, those Professor. parasitize. As such, leafy mistletoes are that have been linked to European Department of Horticulture, University of Wisconsin, Madison, WI53706 Research conducted at the Univer- considered a pathogen of woody plants, cultural traditions, are V. album L., the sity of Wisconsin, Madison. All artwork ©1992 by L.K.P. and in some situations they can cause “original” Christmas mistletoe, and 324 HortTechnology ž July/Sept. 1992 2( 3) several Phoradendron species, which as a part of secular celebrations. As Hawksworth, 1961). The idea behind became a substitute for Viscum among Christianity superseded early pagan the purported differences between Europeans transplanted in the New religions, traditions involving the mistletoes from different hosts is based, World. Another genus of importance mistletoe gradually became incorpo- it turns out, on fact. It has been found in North America is Arceuthobium— rated into Christmas celebrations that mistletoes accumulate compounds the dwarf mistletoe—which, unlike the (Calder, 1983). For a time, there was that are produced by their hosts; leafy mistletoes, causes severe damage a Christian legend that the mistletoe mistletoes growing on coffee trees con- in coniferous forests of the western plant was once a forest tree. When tain caffeine in their tissues (Kuijt, United States. Jesus was crucified, the mistletoe was 1969). The role of mistletoe in the only species that would allow itself In modern times, the medicinal to be used for the making of the cross. value ofmistletoe has been questioned. human culture In punishment, the mistletoe was con- Scientists in the early part of the 20th Folklore and the Christmas tra- demned to spend its life as a lowly century tested some of the traditional dition. The earliest known mention of parasite of other trees. It became known uses for this material and, as one might mistletoe is in the writings of the Greek as “l’herbe de la croix” (wood of the expect, found many of the believed philosopher Theophrastis (Kanner, holy cross), but its association with the curative powers of the plant were with- 1939) in the 3rd century BC. Theo- crucifixion eventually enhanced its sta- out foundation. However, alkaloids phrastis’ detailed description of its mor- tus among Christians, and it became a derived from V. album have been found phology, parasitic growth habit, and tradition in many European countries to be somewhat effective against sev- variation within the family indicates to make rosaries from the wood of the eral circulatory ailments such as heart the Greeks had a well-developed un- mistletoe (Kanner, 1939). disease, hypertension, arteriosclerosis, derstanding ofmistletoe. The dispersal Mistletoe as a medicinal plant. and hemorrhage. Some work has also of mistletoe seeds by birds was ob- Many cultures have used mistletoes for been done involving Viscum com- served and recorded by Pliny, who medicinal purposes. Use of various pounds that may act as anticancer wrote in the first century AD: “In what- species has been documented among agents (Gill and Hawksworth, 1961; ever way the seed is sown, it will never the Navaho, the Japanese, Malaysians, Kuttan et al., 1990). come to anything, unless it has been and native Australians, in addition to Mistletoe and fertility. Many first swallowed and then voided by the well-documented Indo-European herbal folk traditions reflect character- birds...” (Kanner, 1939). And while lore (Gill and Hawksworth, 1961). istics of the plant’s growth habits. Be- this observation is not true, the phe- Like many medicinal plants used in the cause mistletoe is able to bear fruit nomenon is common enough to have past, mistletoe contains compounds even in winter, a number of cultures given the plant its name. Our word that in fact do have physiological ef- came to associate it with fertility. The “mistletoe” is derived from the fects on humans. Additionally, the plant Ainu of Japan sprinkled chopped Anglo-Saxon “misteltan,” “mist” contains compounds that are toxic to mistletoe leaves on their fields to en- meaning dung and “tan,” twig (Calder, humans and animals. For this reason, sure a good crop. Welsh farmers be- 1983). along with recipes for potions and lieved that a healthy crop of mistletoe Folklore traditions involving mis- medicines, cures for mistletoe poison- foretold a good crop the following tletoe developed among several Indo- ing are found in early herbals. One season (Gill and Hawksworth, 1961). European peoples, and our modern such antidote prescribed a mixture of Mistletoe was thought to influence traditions seem to be derived mainly rue, resin, and castor taken in warm human fertility as well and was prescribed from those of the Celts.