[Arceuthobium spp.][Bayer code: none]
American or true mistletoes [Phoradendron spp.][Bayer code: none]
[Viscum album L.][Bayer code: none][CDFA list: B]
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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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.
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.
and SEEDS: Berries shiny, +/- gelatinous, mature in ~ 2
seasons, contain 1(2) seeds surrounded by adhesive tissue
depends on host species.
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.
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
The mistletoes are distinct and unlikely to be confused with any other species.
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.
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