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. Called “the vinegar (Kanner, 1939). to people who had fertility problems of Celestial Tree” by these early people of Despite the risks involved in its all kinds. In Austria, a sprig of mistletoe the British Isles, the mistletoe plant use, medical practitioners of many cul- was placed in a couple’s bed to encourage was thought to be of divine origin, tures used mistletoe as a curative for a conception. It was made into a salve for being rooted, as it is, closer than all number of ailments (Fig. 1). At vari- treatment of male impotence and was other plants to the heavens. The Druids, ous times, it has been used as a seda- consumed by women with wine to en- holy men of Celtic society, reportedly tive, to stop hemorrhaging and diar- hance fertility, and many cultures have performed a number ofrituals involving rhea, and as a treatment for pleurisy, used mistletoe to induce labor or the mistletoe, and our mistletoe traditions gout, heart disease, syphilis, and epi- expulsion of afterbirth. It was also are thought to arise from these early lepsy. It was used externally for sores, thought to be a powerful abortifacient practices (Kanner, 1939). swellings, ulcers, and abscesses, applied and has been used to terminate un- The origins of these traditions to frostbitten limbs, and used to draw wanted pregnancies (Kanner, 1939). predate the advent of Christmas and out “humors,” thought to be the cause Native American women in Texas and are linked to pagan ceremonies involv- of disease (Kanner, 1939). For some Oklahoma still use mistletoe for birth ing the winter solstice. These rites illnesses, the prescription was for control and pregnancy-related problems were thought to ensure the return of mistletoe of a particular tree: mistletoe (Meadows, personal communication). the sun’s warmth in the spring. Mistle- on willow for St. Anthony’s fire and Mistletoe as a food source. As an toe was seen as a symbol of this cycle— powder of mistletoe of oak to be taken evergreen that fruits in winter, mistle- a green, growing sign that summer daily as a preventive for the bubonic toes of several genera are important would indeed return. It was within this plague. Native American women drank food sources for wildlife in many parts context that mistletoe became an im- a tea made of Phoradendron juniper- of the world. Birds eat the berries, and portant part of the Druid winter sol- inum and juniper twigs as a muscle the foliage is a source of winter forage stice and New Year ceremonies, as well relaxant in childbirth (Gill and for deer in the southwestern United

HortTechnology ž July/Sept. 1992 2(3) 325 system. When it dries, the viscin, com- posed of cellulolytic strands in a pectin matrix, is strong enough to keep rain from washing the seed off the branch. Until quite recently, farmers used this adhesive quality to ensnare blackbirds and other avian pests. Mistletoe ber- ries were collected and used to make bird lime. The birds were baited with food and became trapped in the sticky viscin when they landed to feed (Gill and Hawksworth, 1961; Kanner, 1939). The mistletoe seed commonly undergoes a dormant period of up to 6 months before germinating. This dor- mant period is thought to be induced

by elevated CO2 levels within the peri- carp of the berry and is broken when the pericarp is opened and the seed exposed to the air (Lamont, 1983). Germination and establishment of the mistletoe plant on its host are slow processes, taking up to 6 weeks. The mistletoe is thought to maintain itself during this period through pho- tosynthesis; the embryo of mistletoe seeds contains functional chloroplasts (Salle, 1983). As the seed germinates, the hypocotyl grows out parallel to the adjacent surface of the tree (Fig. 3, top). After growing a short distance (usually less than half an inch), it curves inward toward the tree, guided by Fig. 1. An illustration of leafy mistletoe on an oak tree from an early herbal. Adapted from Hortus sanitatis, publisbed in 1491. negative phototropism (Salle, 1983). When the tip of the hypocotyl makes contact, a structure called the holdfast States. The deer seem to relish this entire crown of a tree. The mistletoes develops. The holdfast, or haustorial food source to the extent that Native of interest here, V. album and Phora- disk, is a cup-shaped structure within American hunters have used it to bait dendron species, are evergreens with which a penetration peg grows. It is them (Gill and Hawksworth, 1961). small ( 1 -inch) waxy leaves and woody composed of very plastic tissues and Many species of mistletoes have been stems (Fig. 2). As perennials, the produces secretions that create an air- used as fodder for domestic animals mistletoes often survive for many years tight seal with the bark of the host. throughout the world, where winter and can continue to grow as long as It is thought that penetration of or drought conditions make standard the host is alive, sometimes becoming the tree bark is accomplished through forage plants unavailable (Gill and quite large. both mechanical and enzymatic means Hawksworth, 1961). Mistletoe seeds form in the pearly (Fig. 3, middle). Secretions of Mistletoe also provides a winter white “berry” with which we have pectolytic enzymes dissolve the middle source of nectar and pollen for honey become familiar at Christmas time. lamella between host cells, then pres- bees across the southern and western Botanically, the fruit is not considered sure from haustorial growth pushes United States (Gill and Hawksworth, a true berry. It is called a “pseudoberry” the tissues apart and allows entry (Salle, 1961). and contains a single seed. The seed is 1983). Once inside the tissues of its wrapped in a sticky substance called host, the haustorium grows through Horticultural roles of leafy viscin and enclosed in a rather leathery cortical tissue and phloem until it mistletoes pericarp. Viscin is a major factor in the reaches the xylem. Life cycle of leafy mistletoes. The dispersal and survival of the mistletoe The mistletoe derives its nourish- mistletoes are a diverse group of spe- seed. Dispersal in most species is ac- ment from the xylem. Several theories cies that vary greatly in growth habit complished by birds, for whom the have been proposed to explain why the and physiology. They range from tiny, berries are an important winter food leafy mistletoes have evolved to para- highly parasitic dwarf mistletoes that source. The mucilaginous viscin glues sitize their hosts in this way, passing form small tufts of leafless shoots with the seed to the branch where it has through the much more nutritive ph- inconspicuous flowers to large, woody been deposited after passing, unaf- loem tissues to tap the nutritionally epiphytes capable of supplanting the fected, through the bird’s digestive deficient xylem. Some researchers feel

326 HortTechnology ž July/.Sept. 1992 2(3) that it may be due to a wound response tissue is usually present in leafy mistle- shrubby appearance, with many in the host phloem that does not allow toe haustoria, it is not directly con- branched stems arising from a central establishment of the parasite in these nected with host cells. Once a sinker is point. It can also spread aerially through tissues (Knutson, 1983). Other theo- formed, the parasite may be considered runners called extracorticular strands, ries involve mistletoe adaptation; per- established within its host. Further which then penetrate the host at a haps the evolutionary development of growth of this endophytic portion of point some distance from the mother the haustorium from root tissue pre- the mistletoe occurs only in host cor- plant and form a new plant. Mistletoes cludes the ability to tap phloem tissue tical tissue (Fig. 3, bottom). Intra- mature slowly and do not produce (Knutson, 1983). Finally, it has been cortical strands spread laterally within flowers for up to 5 years after establish- theorized that the xylem is tapped so the cortex and additional sinkers form ment. Flowers of Viscum and Phora- that the mistletoe can obtain vital hor- at intervals to tap the xylem tissue. The dendron are small, sessile, and incon- mones such as the cytokinins, which parasite can exist for a long time spicuous, usually yellowish-green. Male are generally produced in the roots of endophytically before any shoots or and female flowers are borne on plants and are produced in insufficient leaves are formed. Extreme examples separate plants, and female flowers are quantities by the parasite (Kuijt, 1969). of this occur in the dwarf mistletoes, thought to be pollinated by wind or Once it reaches the xylem, the which obtain photosynthate as well as insects. Time of flowering varies with haustorium forms a “sinker,” a wedge- water and minerals from their hosts. In species: European mistletoes flower in shaped cluster of cells that penetrates these plants, aerial portions are neces- spring, American Phoradendrons in fall. the xylem tissues of the host to extract sary only for reproduction. The berries of most temperate species nutrients and water. The nature of the In the leafy mistletoes, aerial tend to ripen in winter, regardless of connecting structures between para- growth is necessary for photosynthesis flowering time, and thus the cycle site and host is not well understood. to proceed. Although they can survive begins again. These connections seem to lead directly long periods as endophytes, develop- The disease process. Mistletoes vary from host xylem to parasite xylem ment of stems and leaves is the norm. in size and degree to which they para- (Salle, 1983). And although phloem Gradually, the mistletoe takes on a sitize their hosts. Both V. album and the Phoradendrons have broad host ranges encompassing several hundred host species, including both decidu- ous trees and conifers. Viscum is found in most of Europe, except the north- ern and western areas of Scandinavia above the 60th parallel. It grows most often on poplar, pine and fir in for- ested regions. In addition, it is a major pest in many fruit-growing areas, par- ticularly on apple, almond, and cherry (Knutson, 1983; Salle et al., 1984). When Europeans first came to North America, Phoradendron was found mainly on hardwoods such as elm, hickory, beech, poplar, walnut, and cherry. Today it causes some dam- age on native trees in the western part of its range (southcentral United States) and in citrus and nut orchards across the South and in California (Knutson, 1983). Both genera are considered pests where they occur on ornamental plantings in cities and parks (Han et al., 1988; Michailides, 1987). The characteristics of the mistle- toe-host relationship differ from those of the typical pathogen–host interac- tion. Obviously the leafy mistletoes bear little physical resemblance to fun- gal or bacterial pathogens. More im- portantly, whereas many plant patho- gens derive their sustenance by con- suming host tissues, mistletoes sustain themselves without major disruption of host functions, simply making use of the host’s vascular system to provide Fig. 2. Mature leafy mistletoe plant established on a host branch. for their survival. Because of this, many

HortTechnology ž July/Sept. 1992 2(3) 327 crease in cambial activity; in particular, production of xylem cells increases. This causes hypertrophy of the tissues immediately adjacent to the mistletoe infestation and can lead to disruption of continuity in the cambium. The amount of abnormal wood formed is proportional to the amount of hausto- rial spread of the mistletoe. Another common characteristic in mistletoe infestation is the dieback of the in- fested limb above the point of attach- ment. This has long been thought to be simply a result of the diversion of water and nutrients by the mistletoe and starvation of the host branch. However, recent studies have shown that the mistletoe actively draws nu- trients from this portion of the branch (Knutson, 1983). Dieback not only allows for-increased nutrient uptake by the mistletoe, but it can benefit the parasite by creating an opening in the canopy, resulting in more light pen- etration. When several mistletoes have es- tablished themselves on a single tree, a condition called spiketop may develop, killing the entire host tree crown. The condition is not common in leafy mis- tletoes; it is most frequent with dwarf mistletoes due to the much higher metabolic drain that these more highly parasitic plants cause. Witches’ broom is another symp- tom that occurs with leafy mistletoes but more often with dwarf mistletoes. Normally dormant adventitious buds located below the point of infestation are stimulated and begin to elongate. They tend to remain rather physiolog- ically immature, providing young tis- sue for the mistletoe to invade and drawing large amounts of nutrients and water into the area of the infesta- tion. The diversion of nutrients and metabolites to these witches’ brooms Fig. 3. Stages of leafy mistletoe germination and development. (Top) Deposition on a twig, is a factor in the occurrence of spiketop germination of the mistletoe seed (Middle) Development of the haustorium and penetration of host by the primary sinker. (Bottom) Development of endophitic tissue in host xylem. and dieback. These symptoms are most extreme in the highly parasitic dwarf mistletoes. In leafy mistletoe infesta- of the symptoms they cause tend to be In general, young host tissue is tion, the host tree will often live indefi- rather generalized. The mistletoe cre- more susceptible to mistletoe infesta- nitely in a stressed but not seriously ates a drain on host resources that re- tion than old tissue, simply because it weakened condition. Because the duces growth, decreases vigor, and is more easily invaded. Old wood, when mistletoe draws water and nutrients correspondingly increases susceptibil- infested, becomes rubbery and weak- from its host, metabolic activity in the ity to other disease and insect pests. ened and is more susceptible to break- host generally increases. Nutrient flow Morphological symptoms also age. In all cases, the weight of the is unbalanced as nutrients accumulate result from this sequestering of re- mistletoe plant can break limbs and in the tissues surrounding the infesta- sources or from alteration of hormone encourage establishment of microbial tion site. How the mistletoe coordinates balances, and they include localized pathogens. these changes is unknown, but the host hypertrophy, dieback, spiketop, and As the endophyte becomes estab- hormones are thought to adjust to witches’ brooming. lished, the host responds with an in- achieve them (Knutson, 1983). One

328 HortTechnology ž July/Sept. 1992 2( 3) change that has been observed is a de- dormant season (Michailides et al., mistletoe infestation in orchards and crease in cellular Ca in tissues surround- 1987), but this method must be con- ornamental plantings in Sonoma ing the infestation, and this is thought to sidered more of a therapy treatment. County was not a native Phoradendron increase cell membrane leakiness and The endophytic portion of the parasite species, but was, in fact, V. album —an therefore the flow of nutrients into the is usually not killed, and after a setback exclusively Old World genus. This was mistletoe (Knutson, 1983). of a year or two, the mistletoe regrows. extremely puzzling; it represented the In summary, the leafy mistletoe There has been some experimen- only known instance of introduction survives by sequestering nutrients and tation with injecting carefully calcu- of a mistletoe from one continent to water from the host’s vascular system. lated doses of herbicide into the trunk another (Scharpf and McCartney, The parasite accomplishes this by hav- of the host tree. As might be expected, 1975). It was theorized that Viscum ing a higher osmotic potential and a the possibility of damage to the host is had been brought into the country in- higher transpiration rate than its host great. In one study, with a mistletoe advertently on apple seedlings, as apple (Schulze and Ehleringer, 1984). Symp- that parasitizes eucalyptus, injected 2, 4- is one of the most common hosts of toms of mistletoe parasitism are both D killed 70% to 100% of the mistle- this species in Europe. Scientists found direct and indirect results of this process. toes, but also partially defoliated all the the situation to be a good opportunity Control of leafy mistletoes. In trees and 5% of the trees did not sur- to observe the colonization of a habi- general, leafy mistletoes are not con- vive the treatment (Greenham and tat by a newly introduced species, and sidered a major pathogen of woody Brown, 1957). they were able to use ecological models plants in North America. However, Other possible control methods to determine the center of its range., they can become a problem in local- include biological control and breed- The probable point of introduction ized areas, especially if they become ing of host trees for resistance. Work was -found to be in the city of Sebas- established in orchards, as well as in concerning biological control agents topol, and by calculating the rate of ornamental plantings such as city parks for mistletoes appeared in the litera- spread, it was determined that Viscum (Han et al., 1988; Knutson, 1983; ture as early as 1915 (Weir, 1915) was introduced into the area around Michailides et al., 1987). Once even a when a fungal parasite of dwarf 1900 (Scharpf and McCartney, 1975). few mistletoes are established, the birds mistletoe was described. However, to After 10 years ofintermittent work that vector mistletoes can exacerbate date, none of the many fungal or insect on this project, it occurred to two the problem. Because the birds tend to pests of mistletoes have been identi- researchers, R. Scharpf and F. Hawks- roost in the tops of the trees where fied as having potential to control any worth, that at the center of the area of they are feeding on the berries, a num- of the mistletoes. infestation might be the answer to the ber of mistletoes can infest a single tree Work on host resistance is just puzzle. Sebastopol was the home of and seriously weaken it. When mistle- beginning. In Europe, variation in sus- Luther Burbank, a horticulturist known toe does become a problem, options ceptibility to Viscum infestation among for his work in plant breeding and for for control are limited. Any control Populus populations has been noted, his interest in novelties of the plant method used on the mistletoe has the but no breeding work has been done kingdom. Indeed, upon searching Bur- potential of having an effect on the (Salle et al., 1984). The resistance bank’s extensive records, they found host, because mistletoe, like its host, is mechanisms that have been observed that the European mistletoe was one a higher plant. appear to be of the preformed or static of the many species that he studied. He Several approaches have been type, involving physical barriers to had apparently obtained Viscum seed taken in efforts to control leafy mistle- penetration. Damage from leafy from J.C. Vaughan of Chicago with toes, but to date, the most commonly mistletoe infestation can be a serious the intent of growing them for Christ- used and most effective method re- problem and development of success- mas decoration (Scharpf and Hawks- mains simple pruning of the limb be- ful control programs is important in worth, 1976). Today, the range of this low the point of infestation. This can affected areas. Eradication of existing species in North America is spreading be an economical and fairly efficient parasites is problematic. Efforts should slowly from its center at Sebastopol method, particularly where pruning is focus on reducing inoculum and (Hawksworth and Scharpf, 1986). Its a part of regular maintenance, as in an avoiding new infestation. Where pos- presence there illustrates the dual role orchard. It has been found that in sible, resistant tree cultivars should be that mistletoe plays in our lives—both other situations, such as in city parks, used. Some work has been done with as a weedy pest and as a valued part of the cost of this method can be pro- hormone sprays (Han et al., 1988), in our cultural heritage. hibitive. In the late 1970s the city of efforts to cause flower abscission or It is not known whether Burbank Sacramento, Calif., was spending an interfere with fruit set. A timely herbi- ever marketed the mistletoe that he estimated $1.4 million/year on prun- cide spray might also interrupt the propagated at Sebastopol. Today, the ing of mistletoe-infested street trees mistletoe reproductive cycle. Com- mistletoe sold in florist shops at (Michailides et al., 1987). bining these practices with pruning Christmas time is gathered in the wild, The search for a cheaper, more- may facilitate control of the problem not cultivated. The gathering and effective control method has been on- in orchard and ornamental settings. marketing of mistletoe is a small but going since the 1950s. Chemical An immigrant in thriving enterprise for a handful of control has been investigated, and sev- family-owned companies in Texas, eral compounds as well as several California: Producing Oklahoma, and California. All have methods ofapplication have been tried. mistletoe for Christmas well-established business, spanning Researchers had some limited success In the mid-1960s, researchers in decades, and in some cases genera- with a spray of 2,4-D during the host northern California discovered that a tions. One company operates in Tulsa,

HortTechnology ž July/Sept. 1992 2(3) 329 Okla. Thirty-five to 40 employees are P. Bernhardt (eds.). The biology of mistle- Michailides, T.J., J.M. Ogawa, J.R. hired for the briefpicking season, which toes. Academic, Sydney. Parmeter, Jr., and S. Yoshimine. 1987. lasts from Thanksgiving through Survey for and chemical control of leafy Gill, L.S. and F. G. Hawksworth. 1961. The mistletoe (Phoradendron tomentosum subsp. mid-December. Mistletoe is collected mistletoes: A literature review. U.S. Dept. from street trees and private properties macrophyllum) on shade trees in Davis, Agr. Tech. Bul.. 1242. U.S. Government California. Plant Dis. 716):533-536. right in the city. Teams of pickers and Printing Office, Washington D.C. baggers work in 12-h shifts around the Salle, G.C. 1983. Germination and estab- clock. They are paid by the pound or Greenham, C.G. and A.G. Brown, 1957. The lishment of Viscum album L., p. 145-159. control of mistletoe by trunk injection. J. In: M. Calder and P. Bernhardt (eds.). The by the bag. The packaged mistletoe is Austral. Inst. Agr. Sci. 23:308-318. sold to wholesale florists across the biology of mistletoes. Academic, Sydney. United States and Canada, and small Han, S., A.M. Berry, and M.S. Reid. 1988. Salle, G., A. Armillotta, and H. Frochot. amounts are exported overseas New ways to control mistletoe. Proc. Calif. 1984. Mechanisms of resistance of four (Meadows, 1992). Weed Conf.. 10:42-45. cultivars of poplar against Viscum album Conclusions Hawksworth, F.G. and R.F. Scharpf. 1986. L., p. 22-30. Proc. 3rd Intl. Symp. Para- Spread of European mistletoe (Viscum al- sitic Weeds, Intl. Ctr. Agr. Res. in Dry Since early times, humans have bum) in California, U.S.A. European J. Areas, Aleppo, Syria. recognized and appreciated the un- For. Pathol. 16(1):1-5. usual relationship of the mistletoe with Scharpf; R.F. and F.G. Hawksworth. 1976. the trees it lives on. To our ancestors, Kanner, L. 1939. Mistletoe, magic and Luther Burbank introduced European medicine. Bul. Hist. of Med. 7(8):875- mistletoe into California. Plant Dis. Rptr. the special characteristics of this plant 60(9):740-742. endowed it with mystical powers. For 936. plant scientists, from Theophrastis to Knutson, D.M. 1983. Physiology of Scharpf, R.F. and F.G. Hawksworth. 1974. Luther Burbank, the mistletoe aroused mistletoe parasitism and disease responses Mistletoes on hardwoods in the United wonder and curiosity. Today our study in the host, p. 295-316. In: M. Calder and States. U.S. Dept. Afr. For. Serv. Pest Lflt. ofmistletoe recalls the place this unique P. Bernhardt (eds.). The biology of mistle- 147. family of plants has held in many cul- toes. Academic, Sydney. Scharpf, R.F. and W. McCartney. 1975. tures and can perhaps help us solve Kuijt, J. 1969. The biology of parasitic Viscum album in California—its introduc- some of the mysteries of life and evo- flowering plants. Univ. of California Press, tion, establishment and spread. Plant Dis. lution in the world of plants. Berkeley. Rptr. 59(3):257-262. Schulze, E.-D. and J.R. Ehleringer. 1984. Kuttan, G., D.M. Vasudevan, and R. The effect of nitrogen supply on growth Kuttan, 1990. Effect of a preparation from Literature Cited and water-use efficiency of xylem-tapping Viscum album on tumor development in mistletoes. Planta 162:268-275. Barlow, B.A. 1983. Biogeography of vitro and in mice. J. Ethno. Pharmacol. Loranthaceae and Viscaceae. p. 1-18. In: 29(1):35-41. Weir, J.R. 1915. Wallrothiella arceuthobii. M. Calder and P. Bernhardt (eds.). The J. Agr. Res. 4(4):369-378. biology of mistletoes. Academic, Sydney. Lamont, B, 1983. In: M. Calder and P. Bernhardt (eds.). The biology of mistle- Calder, D.M. 1983. Mistletoes in focus: An toes. Academic, Sydney. introduction. p. 19-46. In: M. Calder and

330 HortTechnology ž July/Sept. 1992 2(3)