Ponderosa pine, lodgepole pine, western larch, Douglas-fir, western
mountain hemlock, rarely western white pine and spruce.
swellings on branches
areas on trunks that may be twice the normal diameter of the tree
1/2- to 6 inch
- Green shoots
protruding from swollen areas on branches and trunks
In response to branch infections, trees often produce "witches'
brooms," abnormal proliferations of many small twigs which
appear as a mass of twigs and foliage. These vary in appearance
from tight clumps to large loose fans. The dwarf mistletoe species
that infect Douglas-fir, ponderosa pine, lodgepole pine, western
larch, and hemlock cause large witches' brooms. Dwarf mistletoe
infections on branches have a spindle-shaped, swollen appearance.
infections may cause the tree to swell to twice its original diameter.
Shoots of the plant protrude from the swellings on branches and
trunks. These are leafless and vary in color and size according
to species; they have different male and female forms on the same
tree. Douglas-fir dwarf mistletoe has 1/4- to 1/2-inch olive green
shoots, while ponderosa pine dwarf mistletoe has 3- to 8-inch olive
green to yellow shoots. Lodgepole pine dwarf mistletoe
has 2.5-inch yellow to olive-green shoots and western larch dwarf
mistletoe has shoots that are dark purple and 1.5 to 4 inches long.
Hemlock dwarf mistletoe has 2-imch shoots that are either green
or red, and true fir dwarf mistletoe has shoots that are yellow-green
and 3 to 8 inches long.
May be confused
with: Elytroderma needle blight on ponderosa pine, simulation brooms
caused by release of suppressed trees on hemlock or lodgepole pine,
brooms caused by viruses on Douglas-fir, fir broom rust.
cycle: Dwarf mistletoe is probably the most damaging disease
of larch and lodgepole pine in Washington, causing growth loss,
wood quality reduction and tree killing. Dwarf mistletoe is a plant,
but is entirely dependent on its host tree. Death of the tree also
means death of the mistletoe, so mistletoes tend to coexist with
their hosts. Dwarf mistletoes are fairly host specific; there is
limited crossover from one species of tree to another. Seeds are
sticky and are forcibly shot from shoots growing on swellings. These
seeds may travel up to 100 feet depending on the species and wind.
When they land on the proper host, they sprout in the spring and
penetrate the thin bark, forming a new infection. Spread is fastest
in multi-storied stands that are somewhat
open; in single-story stands the spread averages 1-2 feet per year.
Birds may be responsible for long-distance spread by carrying seeds
stuck to their feet or feathers. Trunk infections cause swelling
and cracking that permits rot fungi to be introduced into the heartwood.
Trees that are less than 3 feet in height or 10 years in age generally
agents: The classic problems with dwarf mistletoe have occurred
as a result of a multi-storied forest structure, with older trees
raining seed down on younger ones. This is problematic in Eastern
Washington as there are many Douglas-fir/grand fir or pure Douglas-fir
stands that have this type of structure. These stands are typically
regenerated by cutting the overstory and allowing the infected understory
to grow up to form the new crop, thus perpetuating the disease.
Because of the dwarf mistletoe's host specificity, pure stands or
only a few species have more problems than mixed stands of several
Dwarf mistletoe infests 42% of stands of Douglas-fir on the East
Side. While dwarf mistletoe on Douglas-fir is unusual on the west
side of the Cascades, it is a problem on hemlock. In the Pacific
Northwest, it infests 47% of western larch stands, 42% of lodgepole
pine stands, 21% of hemlock stands, and 26% of ponderosa pine stands.
True fir dwarf mistletoe infestations are uncommon in Washington.
The major impact of dwarf mistletoe infestation is in growth loss.
An estimated 148 million cubic feet are lost annually in all species
to dwarf mistletoe in Washington and Oregon, and 40 million cubic
feet are lost in Douglas-fir in the Inland Empire.
Choice of silvicultural method depends upon the management goals
for the stand, keeping in mind the stage of the cutting cycle and
the damage being inflicted by the disease. With young stands, the
objective is generally to protect them from infection. With middle-aged
stands, efforts are usually directed towards reducing infection
levels and halting spread. When the stand has reached harvest age,
the objective is to eliminate mistletoe and thus prevent the subsequent
crop from be coming infected. When evaluating the condition of the
stand, determine two things: whether the stand is made up of more
or less than 50% of the host species, and whether more or less than
50% of the trees of the host species are infected. As the infection
may be clumped inside a stand, sometimes it is easier to subdivide
the stand and deal with small areas separately.
These treatments would be appropriate for dealing with dwarf mistletoe
infections in all species:
sanitize. This is a good option if the stand is made up of less
than 50% of the host species, and less than half of the host species
is infected. Remove the diseased individuals to halt the spread.
This can be done by combining sanitizing with a thinning or selective
harvesting operation. It may be necessary to return a couple of
years later to remove trees that were not noticeably infected
until thinning allowed more light. into the stand and stimulated
- Stand conversion.
If the stand is made up of less than 50% of the host species,
but more than half of them are infected, it would be best to convert
the stand to another species by cutting all the trees of the infected
species, including the regeneration. This may be done in a thinning
or partial harvest. Choosing this option depends in part on stocking
levels and stand age, as cutting all the trees of one species
may leave the stand unacceptably stocked; in that case it may
be better to cut the most heavily infected individuals and permit
the lightly infected to stay until rotation age.
If the stand is made up of more than 50% of the host species,
and less than half of the host species are infected, a thinning/sanitation
cut of diseased individuals would be appropriate in a well-stocked
or overstocked stand. In an understocked stand it may be better
to wait until a commercial harvest may be made, especially if
the stand is nearing rotation age.
If the stand is made up of more than 50% of the host species,
and more than half of them are infected, and the stand is still
quite young (less than 20 years,) clearcutting and starting over
may be the best option. If the stand is middle-aged, it may be
best to cut the most seriously infected trees and leave the more
lightly infected until rotation age.
- Do nothing.
If the host species is more than 50% of the stand, and more than
half of the trees are infected, a sanitation cut of diseased individuals
may leave stocking levels too low. In that case, if the trees
are nearly merchantable, it is better to do nothing.
a stand, a seed tree or shelterwood cut may be used if the overstory
is removed before the young trees are greater than 3 feet in height
or have reached 10 years of age, as they will probably escape infection.
Delaying removal of the overstory, however, can seriously jeopardize
the new stand. It is beneficial to establish a single-storied, mixed-species
stand when regenerating as that sharply slows the spread of mistletoe
from surrounding stands, and will also help manage some problems
such as root diseases and damaging insects. Combining a seed tree
cut with planting may be an inexpensive way to get a mixed-species
stand. Lastly, if the stand to be regenerated is surrounded by dwarf
mistletoe infected trees, and the species desired is the same species
as the infected trees, a buffer strip wider than the projectile
range of the seeds should be planted with another species around
the perimeter of the new stand.
J.W. Schwandt, Dwarf mistletoe management strategies for inland
Douglas-fir and Grand fir types. In: Silvicultural management strategies
for pests of the interior Douglas-fir and Grand fir forest types,
proceedings of a symposium held February 14-16, 1984 and available
from WSU Cooperative Extension.