Spread and Intensification

Spread is movement of mistletoe from one
tree to another; intensification is the
increase of mistletoe populations within a
tree. Nearly all Douglas-fir dwarf mistle-
toe spread is local and results from explo-
sive discharge of seeds. Wind exerts a
minor influence on distance and direction
of seed travel. Birds and other animals are
responsible for some long-distance spread
when seeds that stick to them are later
rubbed off onto susceptible trees.

Several factors affect spread of Douglas-
fir dwarf mistletoe. These include stand
structure, tree size, species composition of
stands, tree spacing, infection position,
and topography. In single storied stands,
horizontal spread averages 1.5 to 2 feet
(0.5-0.6 m) per year. Spread in multisto-
ried Douglas-fir stands is more rapid
because understory firs are showered with
dwarf mistletoe seeds from infected over-
story trees. Seeds discharged from plants
high up in tree crowns travel farther than
those originating from lower positions.

Presence and arrangement of nonsuscepti-
ble tree species can slow the spread of
Douglas-fir dwarf mistletoe. Spread rates
in dense stands are less than in more open
stands because dwarf mistletoe seed pro-
duction is usually lower due to more shad-
ing and poorer host vigor, and many seeds
are trapped within the foliage of the
infected trees rather than escaping to adja-
cent trees. Spread tends to be more rapid
from ridgetops than narrow valley bot-
toms. Douglas-firs less than 4 feet (1.2 m)
tall are small targets for seed deposition
and have low infection rates, even though
trees of all sizes and ages are susceptible .

The 6-class dwarf mistletoe rating (DMR)
system is commonly used to categorize
intensity of infection in Douglas-fir trees
and stands. For this system, the live crown
of the tree is visually divided into thirds
and each third rated as: 0 = no visible

infection, 1 = light infection (less than half
of the branches infected), or 2 = severe
infection (more than half of the branches
infected). The three ratings are then added
to obtain a tree rating.

A tree severely infected in each third
would be rated class 6. Because Douglas-
fir dwarf mistletoe shoots are small and
often difficult to observe, particularly in
large trees, witches’ brooms are usually
used as an indication that a branch is
infected when determining the rating for
each crown third. The tree ratings of all
live trees (including uninfected ones) are
averaged to obtain a stand or plot rating.

Douglas-fir dwarf mistletoe infection
increases about one DMR class per decade
for individual trees, however this can be
highly variable. Infection intensifies most
rapidly in saplings and pole- size trees
under infected larger trees in stands with
openings. Mistletoe populations increase
within trees as a result of continued depo-
sition of seeds from adjacent infected trees
and from seeds produced by plants in the
trees.

Most initial infections are located in the
lower portions of crowns because this
material is older and has been exposed to
infection longer and represents a larger
surface area for seeds to strike. Infections
resulting from these initial plants tend to
spread upward in the crowns at a rate of 4
to 6 inches (10-15 cm) per year.

Effects of Infection

Infection of Douglas-fir by dwarf mistle-
toe alters tree form, reduces vigor and
growth rates, reduces seed production,
increases susceptibility to other damaging
agents, and results in topkilling and tree
death. These effects result from the para-
site taking food and water from the host,
thus reducing the amount of energy avail-
able for the tree’s normal growth, repro-
ductive, and protective processes.


Pesticides used improperly can be injurious to
humans, animals, and plants. Follow directions
and read all precautions on the labels. Consult
your local forest pathologist, county agricultural
agent, or State extension agent about restrictions
and registered uses of particular pesticides.

The United States Department of Agriculture (USDA) prohibits discrimination
in its programs and activities on the basis of race, color, national origin, gender,
religion, age, disability, political beliefs, sexual orientation, and marital or fami-
ly status. (Not all prohibited bases apply to all programs.) Persons with disabili-

ties who require alternative means of communication of program information

(Braille, large print, audiotape, etc.) should contact USDA’s TARGET Center

at (202) 720-2600 (voice and TDD).

To file a complaint of discrimination, write USDA, Director, Office of Civil
Rights, Room 326-W, Whitten Building, 14th and Independence Avenue, SW,
Washington, DC 20250-9410 or call (202) 720- 5964 (voice or TDD). USDA is
an equal opportunity provider and employer.

R6-NR-FID-PR-003-00



Some rodents, such as porcupines and
squirrels, feed on bark tissues at infection
sites because of the accumulations of
starch and nutrients at these locations. The
large witches’ brooms caused by the para-
site are used for hiding, thermal cover, and
nesting sites by grouse, hawks, owls,
squirrels, porcupines, martens and other
wildlife. Northern spotted owls east of the
Cascades show an attraction to Douglas-fir
witches’ brooms for nest sites.

Management

Management of Douglas-fir dwarf mistle-
toe should consider the role that it plays in
the tree, stand, and overall ecosystem in
which it resides. Douglas-fir dwarf mistle-
toe can serve as an important disturbance
agent, food for other species, and provide
habitat for a variety of organisms. It is a
“pest” only when it interferes with accom-
plishment of management objectives.

In stands where good timber production or
intensive recreation use are major consid-
erations, control of dwarf mistletoe may be
necessary in order to meet management
objectives. However, in areas where tim-
ber production or high recreation use are
not the principal concerns, Douglas-fir
dwarf mistletoe may not adversely influ-
ence management objectives and actions
could be developed to maintain or possibly
increase Douglas-fir dwarf mistletoe popu-
lations for wildlife habitat.

The presence of Douglas-fir dwarf mistle-
toe can thwart accomplishment of tree,
stand, and forest management objectives.
When this occurs, control of the parasite
should be considered. Many traits of
Douglas-fir dwarf mistletoe can be used to
develop successful management practices:
1) the parasite is host specific, essentially
only infecting Douglas-fir, 2) it is an
obligate parasite that requires a living host
for its survival, 3) spread and intensifica-
tion are slow and predictable, 4) it is more

vigorous in full sunlight than under shade
and 5) the disease is rather easily detected
because it causes distinct symptoms, the
witches’ brooms.

Douglas-fir dwarf mistletoe can be man-
aged with cultural treatments that change
the vegetative conditions of the tree, stand,
or forest. No practical chemical or biologi-
cal controls are available for treating
infected trees and stands.

Good timber volume production is almost
impossible in multi-storied, infested
Douglas-fir stands. Treatments designed
to produce stands with a single canopy
layer of Douglas-firs offer the best
prospects of preventing unacceptable loss-
es to dwarf mistletoe.

The most effective methods for managing
Douglas-fir dwarf mistletoe in timber-pro-
ducing stands involve killing of all infect-
ed trees by cutting (harvesting) and/or
burning. Infected trees can be girdled and
left standing if they are needed for snags.
Dwarf mistletoe plants in a single over-
story tree with a crown diameter of 30 feet
(10 m) could cast seeds over 6,400 square
feet (0.15 acre or 0.06 ha).

To slow invasion of young Douglas-fir
stands by dwarf mistletoe from infected
border trees, the ratio of perimeter to area
of treatment sites should be as low as pos-
sible. They should not be long, narrow
strips. Advantage should be taken of any
potential barriers to dwarf mistletoe
spread, such as roads, treeless ridge-tops,
natural openings, and changes in timber
types when laying out the boundaries of
treatment units.

Where clearcutting of dwarf mistletoe-
infested Douglas-fir stands is not appropri-
ate, shelterwood and seed tree harvests can
be effective alternative even-age manage-
ment methods. Trees selected to provide
shelter or seeds should be uninfected or
only lightly infected (DMR less than 3).


The effects of Douglas-fir dwarf mistletoe
infection are progressive. Lightly infected
trees do not differ much from healthy
trees, however they become moderately
then severely infected with the passage of
time, resulting in gradual tree and eventu-
ally stand deterioration.

The effects of dwarf mistletoe on tree
growth increase with severity of infection.
In western Montana it was found that light
infection resulted in a 14 percent reduction
in basal area growth. Medium and severe
levels of infection caused reductions of 41
and 69 percent, respectively. Height
growth was similarly affected.

In the Southwest, severely infected (tree
ratings of DMR 5-6) sawtimber size trees
have 50 to 65 percent less 10-year period-
ic annual volume increment than lightly
infected (DMR 1-2) or healthy trees.
Because severe Douglas-fir dwarf mistle-
toe infection causes marked reductions in
height and radial growth, severely infested
sites look less productive than they actual-
ly are.

Topkill is common in severely infected
trees. Foliage in tops of trees with large
witches’ brooms lower in the crowns grad-
ually become sparse and off-color as the
dwarf mistletoe plants and brooms use
large amounts of water and nutrients need-
ed for tree growth. Eventually the tops
become so weakened they die.

Mortality of Douglas-fir in severely infest-
ed stands (stand ratings of DMR 4 or
higher) is 3 to 4 times greater than that in
comparable non-infested stands in the
Southwest. Even stands with mean dwarf
mistletoe ratings of 2 or 3 show increased
mortality rates over uninfested or lightly
infested (stand DMR 1) stands.

Seedlings and saplings, especially those
with main stem infections, readily suc-
cumb to the parasite. Those weakened by
numerous branch infections or large
witches’ brooms cannot compete success-

fully with surrounding trees and die.

Severely infected trees typically produce
few cones and those that are produced are
smaller than normal. Cones from severely
infected trees weigh only about one-third
of those from healthy trees and have about
half as many viable seeds.

Other forest values are affected adversely
by Douglas-fir dwarf mistletoe. Risk of
wildfires is increased because of the long,
thin twigs in witches’ brooms in the lower
portions of crowns, accumulation of fallen
witches’ brooms around the bases of in-
fected trees, and increased tree mortality.

Witches’ brooms provide fuel ladders for
flames to spread upward into tree crowns.
The small diameter dead twigs in brooms
ignite easily and can serve to spot fires
well ahead of the main fire. Mistletoe
infected trees are less likely to survive
fires than healthy trees.

Large witches’ brooms can increase the
hazard potential in recreation sites because
they are more prone to break and fall than
healthy branches as a result of collecting
snow and ice or presenting large dense
surface areas for wind to blow against.

Dead and dying trees detract from visual
quality. Severely infected trees are less
capable of resisting attacks by other dis-
eases and insects.

There seems to be no strong relationship
with the Douglas-fir beetle (Dendroctonus
pseudotsugae) but severely mistletoe-
infected trees are frequently attacked and
killed by secondary insects, such as the
flatheaded fir borer (Melanophila drum-
mondi).

Benefits of Douglas-fir dwarf mistletoe
have been recognized. The shoots and
fruits are eaten by some birds, especially
grouse. They also serve as a food source
for several species of insects, fungi,
rodents and mammals.


spread of mistletoe to adjacent Douglas-
firs. Witches’ brooms exert a large drain
on infected trees, so their removal by
pruning can improve tree vigor and
longevity.

All branches with witches’ brooms should
be cut. Branches below brooms and two
whorls above them are also likely to be
infected, and should also be pruned, even
if brooms are not obvious. Trees may have
to be repruned occasionally to remove
developing witches’ brooms.

The best candidates for pruning are trees
with infections only in the lower half of
their crowns. In addition to prolonging
tree life, pruning can reduce the danger of
branches and trees breaking and causing
accidents. Pruning branches with large
witches’ brooms should not be done from
January through July to avoid attracting
Douglas-fir beetles to the pruned trees.
Trees too severely infected to be pruned
should be removed if they pose an infec-
tion threat to adjacent Douglas-firs or a
serious safety hazard to users.

There are natural factors which limit
Douglas-fir dwarf mistletoe, including
wildfire which has been nature’s primary
control agent. Several species of insects
and fungi attack and kill dwarf mistletoe
shoots or fruits. Some of the fungi are
common pathogens that can exert consid-
erable local but temporary control of
dwarf mistletoe. Environmental factors,
currently not completely understood, limit
the effectiveness of these fungi to small
areas for short durations.

References

Geils, B.W.; Mathiasen, R.L. 1990.
Intensification of dwarf mistletoe on
southwestern Douglas-fir. Forest
Science 36:955-969.

Hawksworth, F.G.; Weins, D. 1996.
Dwarf mistletoes: biology, pathology,
and systematics. Agric. Handb. 709.
Washington, D.C. : U.S. Department
of Agriculture, Forest Service. 410 p.

Mathiasen, R.L; Edminster, C.B.;
Hawksworth, F.G. 1990. Infection of
young Douglas-firs by dwarf mistle-
toe in the Southwest. Great Basin
Naturalist 50: 67-72.

Mathiasen, R.L.; Hawksworth, F.G.;
Edminster, C.B. 1990. Effects of
dwarf mistletoe on growth and mor-
tality of Douglas-fir in the Southwest.
Great Basin Naturalist 50:173-179.

Pierce, W.R. 1960. Dwarf mistletoe
and its effect upon the larch and
Douglas-fir of western Montana.
School of Forestry Bulletin 10.
Missoula, MT: Montana State Univ.,
38 p.

Tinnin, R.O,; Knutson, D.M. 1980.
Growth characteristics of the brooms
on Douglas-fir caused by
Arceuthobium douglasii. Forest
Science 26(1):149-158.

Weir, J.R. 1918. Effects of mistletoe
on young conifers. Journal of
Agricultural Research 12:715- 718.


Forest Insect
& Disease
Leaflet 54

Revised Febru: 2000

Forest Service

U.S. Department of Agriculture °

Douglas-Fir Dwarf Mistletoe
James S. Hadfield', Robert L. Mathiasen”, and Frank G. Hawksworth?

Douglas-fir dwarf mistletoe, Arceuthobium
douglasii Engelm., is a destructive native
parasite of Douglas-fir and is the primary
disease agent affecting this tree over most

below, its distribution essentially coincides
with that of Douglas-fir from southern
British Columbia to central Mexico. The
parasite is not found in the Douglas-fir

stands east of the Continental Divide in
Montana, most of Wyoming, northern
Colorado, and west of the Cascade Range
(except near the crest) and north of the
Siskiyou Mountains of Oregon. It is the
only dwarf mistletoe that occurs in all of
the western states (Figure 1). The dwarf
mistletoe occurs on many different
Douglas-fir ecologic, successional, and
topographic types.

Douglas-fir is the principal tree species
affected by Douglas-fir dwarf mistletoe.
True firs and spruces are rarely infected
where they are associated with infected
Douglas-firs, but the parasite is of little or
no economic importance on these species.
Douglas-firs are rarely infected by mistle-
toes other than Douglas-fir dwarf mistle-
toe.

of its range. With the exceptions noted

Douglas-fir dwarf mistletoe has increased
in abundance since the late 1800s.
Douglas-fir stands are much more wide-
spread in the Inland West largely as a
result of nearly a century of fire suppres-
sion. Prior to this time, fires restricted the

‘Plant Pathologist, U.S. Department of Agriculture, Forest Service, Wenatchee National
Forest, Wenatchee, WA. ?Assistant Professor, School of Forestry, Northern Arizona
University, Flagstaff, AZ. *Research Forest Pathologist (deceased), Rocky Mountain
Research Station, Fort Collins, CO.


Infected shelterwood or seed trees should
be removed as soon as Douglas- fir
seedlings become established. Failure to
remove infected overstory trees in a timely
manner will contribute to rapid infection
of the developing understory Douglas-firs.

As a general rule, it is desirable to remove
the infected overstory before the Douglas-
firs in the young stand are 3 feet (1 m) tall
or ten years old (whichever comes first).
However, in the Southwest few Douglas-
firs are infected until they reach heights of
4 feet (1.2 m) or 20 years in age.

In mixed species stands that contain
Douglas-firs infected by dwarf mistletoe,
silvicultural treatments should retain other
tree species. Non-hosts left between
infected and non-infected Douglas-firs
prevent or slow spread and intensification
of the parasite.

Sanitation by removal of infected trees can
be an effective treatment in young (10 - 30
years old), lightly infested stands. Lightly
infested is defined as those stands in
which the numbers of desirable dwarf
mistletoe-free trees are sufficient to meet
management objectives. Typically these
stands would have a stand DMR no
greater than 2.

Douglas-firs with one-third or more of
their crowns infected by dwarf mistletoe
generally decline rapidly about 10 years
after they are exposed to full sunlight by
thinning. Because of their rapid decline,
lightly infected trees should not be left
when stands are being sanitized unless
they can be expected to reach mer-
chantable size within 15 years.

Pruning of infected branches, in conjunc-
tion with sanitation, can further reduce
dwarf mistletoe populations, but is seldom
effective in eliminating the parasite
because latent infections will be missed.
Re-examination of stands 5 years after
sanitation is desirable to determine if addi-
tional sanitation is needed. Moderately and

severely infested stands (stand DMR
greater than 2) should not be sanitized
because many trees with latent infections
will be missed, and once these trees are
released from competition, the number of
dwarf mistletoe plants will increase rapid-
ly within their crowns. Such stands should
either be regenerated or left unthinned to
avoid their rapid decline.

Latent dwarf mistletoe infections in trees
in densely stocked stands and in shaded
reproduction are very difficult to detect. It
should be assumed that understory trees
more than 3 feet (1 m) tall that have been
overtopped by infected trees for at least 15
years are probably infected.

Retention of living infected trees, particu-
larly those with large witches’ brooms, for
use as nesting sites and cover for wildlife
is a consideration for some stands. Large
diameter trees with brooms confined to the
lower halves of trees will survive longer
than smaller infected trees overtopped by
large trees.

The risk of losing the stand or large num-
bers of trees to wildfires and other damag-
ing agents can be reduced by clumping the
distribution of infected trees into small
groups separated by much larger areas free
from infection.

Stands that have a uniform distribution of
Douglas-fir dwarf mistletoe will experi-
ence higher rates of tree mortality from
the parasite than those where clumps of
infected trees are widely separated from
each other. Infected trees with large witch-
es’ brooms can be killed by girdling and
left standing for use by wildlife.

Forested recreation sites, such as camp-
grounds, with Douglas-fir dwarf mistletoe
infestations may need to be treated to
maintain Douglas-firs on the sites for
many years and to reduce the risk of
injuries and damage to users from break-
ing branches. Pruning infected trees can
prolong their lives and slow or even stop


system of the parasite becomes more
extensive, the form of the infected branch
or stem becomes distorted.

Douglas-fir dwarf mistletoe typically
becomes systemic within the host when
the endophytic system invades tree buds at
the infection site. Masses of invaded buds
are stimulated to grow and develop long
twigs, which eventually form witches’
brooms. The mistletoe endophytic system
grows apically at the same rate as the
infected twigs. Aerial shoots can be found
on branch growth segments ranging from
2 to7 years old.

pared to healthy branches have very large
surface areas.

Broom development is most rapid in
infected branches exposed to direct sun-
light. Witches’ brooms on lower branches
in dense stands develop slowly and tend to
be small.

Trees may have one to many witches’
brooms. Most extensive development of
witches’ brooms is usually in the lower
half of tree crowns. Brooms can become
so large and numerous that they make up
entire tree crowns (Figure 3). Branches

Fig. 3. Large witches’ broom ina
Douglas-fir.

The most striking symptoms of dwarf
mistletoe infection on Douglas-fir are
witches’ brooms, which are noticeable
within 10 years after infection takes place.
Broom shapes vary but start out as fan-
shaped and eventually tend to become
somewhat spherical.

Branches in witches’ brooms may become
long and droopy and attain lengths of 10
feet (3 m), but are typically only about
one-quarter inch in diameter (these are
locally called “rat tails”). Large brooms
may be several feet in diameter and com-

with big witches’ brooms develop large
diameters at their junction with the main
stem to support the weight, which can be
several hundred pounds. Infected branches
are kept alive because of the ability of the
parasite to obtain food and water from the
tree. In severely infected trees, branches
with witches’ brooms are generally the
last to die.

Severely infested Douglas-fir stands typi-
cally have many trees with stunted
growth, witches’ brooms, dying and dead
tops, and dead trees.


under high internal water pressure, abscises
from its base, contracts rapidly, and explo-
sively propels the seed into the air.

Most seeds are shot upward and outward.
Initial seed velocity is about 72 feet (22 m)
per second. Seeds may travel 30 to 40 feet
(10-13 m), but most land within 10-15 feet
(3-5 m) of the disseminating shoot.

Seeds have a sticky hygroscopic coating
called viscin which enables them to stick to
most objects they strike. Foliage is the
most common receiving surface.

Viscin, when first moistened by rains, acts
as a lubricant. Seeds slide downward and
either fall off needles or become lodged at
the base of needles. Seeds are fastened in
place when the viscin dries and they over-
winter in a dormant state. Many of the
seeds are removed by rain, snow and ice or
are eaten by birds, insects and fungi.

The seeds germinate in early spring.
Radicles grow along twig surfaces until an
obstruction, usually a needle base, is
encountered or the food supply is exhaust-
ed. When radicle extension is obstructed
small mounds of tissue called holdfasts are
formed. Infection pegs that develop from

the holdfasts then infect the host by pene-
trating through the thin bark during the
summer. After the bark has been penetrat-
ed the mistletoe’s root or endophytic sys-
tem grows in the cortex and wood of the
host. Infection occurs most readily in
twigs and stems less than 5 years old.

About 2 to 5 years pass before the first
aerial shoots appear at infection sites.
Infections on branches exposed to direct
sunlight probably produce shoots quicker
than shaded infections. Infections that
have not yet produced aerial shoots are
termed latent infections.

The typical length of time needed for
female plants to complete their life cycle
from initial establishment to dissemina-
tion of the first seed crop is 4 to 6 years.

Symptoms of Infection

The first symptom of Douglas-fir dwarf
mistletoe infection is a slight swelling at
the infection site 1 to 2 years after infec-
tion occurs. Initially these can be difficult
to see. As time passes and the endophytic

Fig. 2. Aerial shoots of Douglas-fir dwarf mistletoe.


Life History

Douglas-fir dwarf mistle-
toe is a small, inconspicu-
ous, seed-bearing parasitic
plant. The aerial system of
this parasite found on
infected branches and
stems consists of slender,
— olive-green, perennial
shoots. They are most
commonly found nestled
7] | among needles on infected
branches. The average
length is only 0.75 in.

(2 cm), about the length of
lm fir the needles, but they

°. are rarely up to 3 inches
rat (8 cm) (Figure 2).
Although shoots contain
¢ chlorophyll, they produce

little food for the parasite.
The major function of the
I aerial shoots is reproduc-

- tion. Individual aerial
oe shoots can live for several
years.

Fig. 1. Distribution of Douglas-fir dwarf
mistletoe in the western United States and
Canada.

distribution of Douglas-fir and dwarf
mistletoes, especially on drier sites domi-
nated by ponderosa pines. Frequent fires
killed the small Douglas-firs, especially
those infected by dwarf mistletoe. Fire
suppression has enabled Douglas-fir to
survive and dominate on millions of acres
where once it was a relatively minor stand
component.

A history of selective harvesting with
removal of more valuable ponderosa pines,
western larch, and large diameter Douglas-
firs has also contributed to the increases in
dwarf mistletoe by leaving small or poor
quality infected Douglas-firs in which the
parasite flourished and spread.

Aerial shoots arise from a network of root-
like strands imbedded in host tissues. This
network, called the endophytic system,
consists of cortical strands growing within
the living bark and sinkers embedded in
the wood. The endophytic system takes
nutrients and water from the host tree.
The endophytic system lives as long as
adjacent host tissues are alive and may
produce aerial shoots for many years.

Male and female flowers are small and
produced on separate plants. Flowering
takes place in May and June. Insects and
wind are both involved in pollination.
Fruits complete their development 17 to
18 months after pollination. Only the
female plants bears the fruits and seeds
that spread the parasite. Berry-like fruits
are olive-green and contain one seed aver-
aging less than 0.1 inch (2 mm) long.

At maturity in September and October, the
elastic outer case of the fruit, which is