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Mistletoe Information
IDENTIFICATION
Leafy mistletoes have green stems with thick leaves that are
nearly oval in shape. Plants often develop a rounded form up to 2
feet or more in diameter. The small, sticky, whitish berries are
produced from October to December. Evergreen clumps of mistletoe
are readily observed on deciduous trees in winter when leaves are
off the trees.
LIFE CYCLE AND BIOLOGY
Mistletoe plants are either female (produce berries) or male
(produce only pollen). The berries of the female plant are small,
sticky, and whitish; they are very attractive to birds such as
cedar waxwings, robins, and others. The birds feed on and digest
the pulp of the berries, excreting the living seeds that stick
tightly to any branch on which they land. In most cases, the
initial infestation occurs on larger or older trees because birds
prefer to perch in the tops of taller trees. A heavy buildup of
mistletoe often occurs within an infested tree because birds are
attracted to the berries, and may spend a good deal of time
feeding on them. In addition, seeds may fall from mistletoe plants
in the upper part of the tree, creating new infestations on the
lower branches. The rapidity with which mistletoe spreads is
directly related to the proximity and severity of established
infestations, and newly planted trees can be quickly infested if
they are growing near old, heavily infested trees.
After the mistletoe seed germinates, it grows through the bark and
into the tree’s water-conducting tissues, where rootlike
structures called haustoria develop. The haustoria gradually
extend up and down within the branch as the mistletoe grows.
Initially, the parasitic plant grows slowly; it may take years
before the plant blooms and produces seed. Broadleaf mistletoes
have succulent stems that become woody at the base. Old, mature
mistletoe plants may be several feet in diameter, and on some host
species, large swollen areas develop on the infected branches
where the mistletoe penetrates. If the visible portion of the
mistletoe is removed, new plants often resprout from the haustoria.
Dwarf mistletoes are smaller plants than broadleaf mistletoes,
with mature stems less than 6 to 8 inches long. Dwarf mistletoe
shoots are nonwoody, segmented, and have small scalelike leaves.
While broadleaf mistletoe seeds are dispersed by birds, dwarf
mistletoe seeds are spread mostly by their forcible discharge from
fruit, which can propel seeds horizontally into trees up to 30 to
40 feet away.
DAMAGE
Broadleaf mistletoe absorbs both water and mineral nutrients from
its host trees. Healthy trees can tolerate a few mistletoe branch
infections, but individual branches may be weakened or sometimes
killed. Heavily infested trees may be reduced in vigor, stunted,
or even killed, especially if they are stressed by other problems
such as drought or disease.
MANAGEMENT
In newly developed areas or in older established areas where trees
are being replaced, the ideal method of controlling or preventing
mistletoe is to plant trees believed to be resistant or moderately
resistant to mistletoe. Avoid trees like Modesto ash, known to be
especially susceptible to mistletoe infestation. Where many new
trees are being planted, control mistletoe in any surrounding
infected trees to reduce the infection of new trees.
For treatment of existing trees it is important to remove
mistletoe before it produces seed and spreads to other limbs or
trees. Mechanical control through pruning is the most effective
method for removal. Growth regulators provide a degree of
temporary control but repeated applications are required. Severely
infested trees should be removed and replaced with less
susceptible species to protect surrounding trees.
Mechanical Control
The most effective way to control mistletoe and prevent its spread
is to prune out infected branches, if possible, as soon as the
parasite appears. Using thinning-type pruning cuts, remove
infected branches at their point of origin or back to large
lateral branches. Infected branches need to be cut at least one
foot below the point of mistletoe attachment in order to
completely remove embedded haustoria. Done properly, limb removal
for mistletoe control can maintain or even improve tree structure.
Severe heading (topping) is often used to remove heavy tree
infestations; however, such pruning weakens a tree’s structure,
and destroys its natural form. In some cases it is best to remove
severely infested trees entirely because they are usually a source
of mistletoe seed.
Mistletoes infecting a major branch or the trunk where it cannot
be pruned may be controlled by cutting off the mistletoe flush
with the limb or trunk. Then wrap the area with a few layers of
wide, black polyethylene to exclude light. Use twine or tape to
secure the plastic to the limb, but do not wrap it too tightly or
the branch may be damaged. In some tree species callus tissue will
form under the plastic, further weakening the limb. Broadleaf
mistletoe requires light and will die within a couple of years
without it. It may be necessary to repeat this treatment,
especially if the wrapping becomes detached or if the mistletoe
does not die.
On large commercial projects this method has
proved to be tedious and time consuming. A different approach is
to spray the wounded area with pruning paint as opposed to
wrapping it. This inhibits light and has also proved to be
effective in killing the plant.
Simply cutting the mistletoe out of an infested tree each winter,
even without wrapping or painting, is better than doing nothing at
all. Even though the parasite will grow back, spread is reduced
because broadleaf mistletoe must be several years old before it
can bloom and produce seed. |
