Dwarf mistletoes [Arceuthobium spp.][Bayer code: none]

American or true mistletoes [Phoradendron spp.][Bayer code: none]

European mistletoe [Viscum album L.][Bayer code: none][CDFA list: B] Photographs


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[GENERAL DESCRIPTION] [SEEDLINGS] [MATURE PLANT] [FLOWERS] [FRUITS and SEEDS] [HABITAT] [DISTRIBUTION] [PROPAGATION/PHENOLOGY] [MANAGEMENT FAVORING/DISCOURAGING SURVIVAL] [SIMILAR SPECIES] [CONTROL METHODS]

GENERAL DESCRIPTION: Widespread shrubby perennial parasites and hemiparasites that grow on the stems of trees and certain shrubs. All mistletoes depend on their hosts for water, mineral nutrients, and to some extent, carbohydrates. Host damage can range from minor swellings to death, depending on the mistletoe species, severity of infection, and health of the host. Trees weakened by mistletoe infections are more susceptible to attack by insects and fungi, which may lead to increased mortality rates. All parts of many species contain toxic amines and may be poisonous to humans and livestock when ingested. Yet, many birds and mammals consume berries and foliage of various species. All dwarf and American mistletoe species occurring in California are native plants. Mistletoes provide food and nesting habitat for many animal species and may be important contributors to the health of natural communities. However, heavy mistletoe infections are undesirable in landscapes, orchards, and managed forest-systems. Identification of individual species of dwarf and American mistletoes can be difficult.

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SEEDLINGS: Cotyledons fused. Only seeds that are deposited on or very near a suitable entry point can survive to infect a host. Following germination, the hypocotyl (radicle in dwarf mistletoe) elongates until it contacts a bud, leaf base, or twig. It then flattens to form a disk-like, adhesive structure or holdfast. A wedge-like organ, primary haustorium, develops from the holdfast and penetrates the twig to just above the cambium layer. Cortical strands grow from the primary haustorium parallel to and just above the cambium layer. At certain points the cortical strands produce secondary haustoria (sinkers) that tap the conducting phloem and xylem. Cortical strands and sinkers continue to grow and comprise the water and food absorbing endophytic system. This initial infection process occurs over a period of ~ 1-2 years. A swelling on the twig at the point of infection is often visible. The first mistletoe shoots grow only a few millimeters from buds on the holdfast. For American mistletoes, first shoots may appear during the first year. First shoots of dwarf mistletoes typically appear during years 2-5.

MATURE PLANT: Leaves opposite.

FLOWERS: Spikes open or interrupted. Flowers inconspicuous, mostly 2-4 mm long, unisexual, male and female on separate plants (dioecious). Ovary inferior. Perianth parts (sepals or petals) minute. Male perianth parts 3-7. Anthers sessile. Flowering months vary with species.

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FRUITS and SEEDS: Berries shiny, +/- gelatinous, mature in ~ 2 seasons, contain 1(2) seeds surrounded by adhesive tissue or viscin.

HABITAT: Variable, depends on host species.

DISTRIBUTION:

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PROPAGATION/PHENOLOGY: Reproduce by seed. Germination does not require the presence of a host and can occur under dry to moist conditions depending on species. Successful infection of a host only occurs when seeds germinate on or near a suitable infection site. Seeds remaining in berries do not germinate, but can survive until berries decompose, about 1 season. Dispersed seed (removed from berries) survives ~ 1 season. Mistletoe shoots loose water through transpiration at much greater rates than host trees and are unable to control their stomata (pores) during drought conditions. Mistletoe tissues can maintain greater concentrations of solutes or osmotic potentials than host tissues. Drought stress occurs in host trees when mistletoe shoots preferentially draw water from the host during periods of water-shortage. The endophytic system can live as long as its host.

MANAGEMENT FAVORING/DISCOURAGING SURVIVAL: In landscapes and orchards, regular pruning of infected branches can control mistletoe growth and seed dispersal. Infected branches should be pruned to at least 30 cm (12 in) behind (proximal to) the infection for American and European mistletoes or to the trunk for dwarf mistletoes. Chemical growth regulators also control growth and dispersal, but do not affect the endophytic system and must be reapplied when mistletoe shoots re-grow. Removing mistletoe shoots can help prevent drought stress in host trees, but may also stimulate the endophytic system to expand and produce new shoots. In some cases, supplemental water or fertilizer and minimizing physical damage and soil compaction can improve tree vigor and longevity. In managed forest-systems, dwarf mistletoe infections spread faster in single species stands, uneven-aged or multi-storied stands, and stands with open canopies. Severely infected trees should be removed.

SIMILAR SPECIES: The mistletoes are distinct and unlikely to be confused with any other species.

CONTROL METHODS:

Prevention and Control: Although currently not a serious problem in California, European mistletoe has the potential to become a widespread pest. It is currently thought that its spread may be limited due to a lack of susceptible host trees surrounding its current distribution. However, longer-range dispersal is very possible through human movement of mistletoe for ornamental or festive purposes. In California, common hosts include species of maple (Acer), alder, (Alnus), apple (Malus), cottonwood (Populus), plum (Prunus), locust (Robinia), willow (Salix), birch (Betula), hawthorn (Crataegus), and elm (Ulmus). European mistletoe does not appear to infect oak, eucalyptus, or conifer species within its current California distribution. Its initial distribution has been very urban in nature, infesting many popular planted tree species in city areas. Pruning infected limbs is the easiest method for mistletoe removal. Limbs should be cut at least 30 cm below the mistletoe shoots. In orchards, this is already a common practice. However, orchards abandoned or sold for urban development may contain sources of mistletoe that are not regularly removed.

References:
McCartney, W. O., R. F. Scharpf and F. G. Hawksworth. 1973. Additional hosts of Viscum album, European mistletoe in California. Plant Disease Reporter 57:904.
Scharpf, R. F. and W. O. McCartney. 1975. Viscum album in California -Its introduction, establishment and spread. Plant Disease Reporter 59:257-262.
Scharpf, R. F. and F. G. Hawksworth. 1976. Luther Burbank introduced European mistletoe into California. Plant Disease Reporter 60:740-742.
Hawksworth, F. G and R. F. Scharpf. 1986. Spread of European mistletoe (Viscum album) in California, U.S.A. European Journal of Forest Pathology 16:1-5.
Hawksworth, F. G, R. F. Scharpf and M. Marosy. 1991. European mistletoe continues to spread in Sonoma County. California Agriculture 45:39-40.

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