Because they have the unique ability to take energy directly
from another plant, the various species of mistletoe can cause the slow, steady
decline of host trees and shrubs.
Mistletoes are perennial, shrubby, woody or semi-woody
flowering plants that attach themselves to other plants and steal water and
nutrients from the host plant. Because
they are dependent upon these host plants for nourishment, they are called
parasites. Unlike dodder, which lacks
any ability to produce food for itself, the mistletoes do contain the green
pigment chlorophyll that allows them to manufacture food from the energy of the
sun.
In the desert Southwest, there are about a dozen different
species of mistletoe that affect trees and shrubs. Each of these species have the ability to
invade the living tissue of host plants and extract water and nutrients for
their own growth and development. It is
this unceasing loss of strength, coupled with the twisting and distortion of
branches caused by the mistletoe that can eventually lead to the decline and
sometimes death of trees and shrubs.
The traditional European mistletoe is Viscum album,
but in the United States two genera, Arceuthobium and Phoradendron
are the representatives of the family. Arceuthobium
species are dwarf mistletoes and are weak, herbaceous plants with leafless
yellow-green to orange stems. These
mistletoes parasitize pines and junipers all across the Southwest.
Members of the Phoradendron generally have
well-developed leaves on strong, shrubby, almost woody stems. Some species have large, yellow-green leaves
while other species are essentially leafless.
The common desert mistletoe that infests many of the desert legume trees
like palo verde, mesquite, and ironwood has only scale-like leaves. The large-leafed yellow mistletoe is often
picked and sold during the holiday season as a way to steal a kiss from someone
special. It is most commonly found on
riparian softwood trees like cottonwood, sycamore, willow and ash.
Most mistletoe species produce small white to pinkish or
green-tinged berries, whose single hard seeds are surrounded by fleshy, sticky
pulp. The fruit seems irresistible to
birds, who then redistribute the seeds to new locations, not only through their
droppings, but also by inadvertently carrying the sticky seeds on their beaks
and feet.
While otherwise healthy host plants may seem to tolerate one
or two mistletoe plants, the parasitic plant’s ability to effectively produce
and disperse seeds all but insures the spread of the infestation and the slow
decline and even death of the host plant.
The weakening, disfigurement and eventual death of shade, food and
lumber producing trees represents a significant worldwide economic loss each
year.
Control of parasitic plants is often difficult. The easiest and most common method of control
is to simply prune or break off the plants.
This is best done before flowering to prevent the development and
dispersal of seeds. Removal of the
parasites also helps to reduce the drain on the host tree as pruning prevents
the loss of valuable water and nutrients from the host.
Unfortunately, mistletoe pruned in this manner usually does
not remove all of the parasitic plant, and the parasite will often grow back
quickly. For this reason, frequent
pruning is often necessary. To prevent
frequent regrowth, remove the mistletoe as close to its point of attachment as
possible. Some bark tissue may safely be
removed during this process, but try not to cut too deeply, or the branch may
snap under its own weight or in a wind.
A more permanent step would be to completely prune away
infected limbs and branches up to 12 or more inches below the mistletoe point
of attachment. This type of pruning works well for younger clumps of mistletoe,
but it cannot be done on older growth mistletoe, or where the mistletoe is on
large, often essential limbs. Be sure to
prune the infected limbs back to their point of attachment with a larger branch
or the trunk. Never leave the stub end
of a branch that could invite the entry of other diseases and wood-destroying
insects.
Where infestations occur on essential branches or on the
trunks of trees, some measure of control can be achieved by pruning the
offending mistletoe back flush with the tree and then wrapping the exposed
points of attachment with dark, light-excluding plastic sheeting. The lack of light on the remaining parasite
tissue will prevent the regrowth of the pest.
As the mistletoe is deprived of sunlight, the parasite will eventually
die. However, this make take a year or
two, so it is important to check the plastic wrap regularly and replace it if
it begins to degrade.
Mistletoe is a common pest on local landscape trees, but by
careful vigilance and proper tree care much of the damage to, and death of,
these valuable plants can be avoided.
If you have questions, you can reach one of the Master
Gardeners at the Cooperative Extension office, 820 E. Cottonwood Lane, Building
C, in Casa Grande. The telephone is
(520) 836-5221, extension 204.
The University of Arizona is an equal opportunity,
affirmative action institution. The
University does not discriminate on the basis of race, color, religion, sex,
national origin, age, disability, veteran status, or sexual orientation in its
programs and activities.
Rick Gibson
Extension Agent, Agriculture
University of Arizona Cooperative Extension
820 E. Cottonwood Lane, Building C
Casa Grande, Arizona 85222
Voice: (520) 836-5221
Fax: (520) 836-1750
email: gibsonrd@ag.arizona.edu
Date: November 26, 2014
