processes. The effects of dwarf mistletoe
on growth increases with severity of infec-
tion and is especially acute in severely
infected trees (DMR=S or 6). Tree volume
growth can be reduced by as much as 40%
in severely infected trees.

Fir live crown ratio has an important effect
on the consequences of fir dwarf mistletoe.
The effect of dwarf mistletoe on radial,
height, and volume growth rates is greatest
in trees with good live crown ratios (>
80%), the fastest growing, and become
almost negligible in trees with live crown
ratios < 40%, the slowest growing. On the
other hand, trees with good live crown

may cause trees and branches to break and
fall. Dead and dying trees detract from
visual quality. Potential for wildfires is
increased because of dead branches,
increased tree mortality, and the accumu-
lation of dead branches around the bases
of infected trees.

Fir dwarf mistletoe infection also can
have beneficial effects. Flowers, shoots,
and fiuits are food for insects, birds, and
mammals. Fir mortality caused by dwarf
mistletoe, either directly or by predispos-
ing trees to other agents, provides snags
as habitat for cavity-nesting birds and,
eventually, coarse-woody debris on the
forest floor.

Management

In forest ecosystems,
dwarf mistletoes have
value as individual,
biological species
and act as distur-
bance agents, influ-
encing both the struc-
ture and function of
forest communities.

Fig. 3. Female shoots of fir dwarf
mistletoe.

ratios but with severe infections grow bet-
ter than uninfected trees with poor live
crown ratios.

Quality of lumber in infected trees can be
reduced because fir dwarf mistletoe often
forms large swellings in the main stems of
true firs. Large knots often are associated
with branches supporting witches’ brooms.
Severely infected trees typically produce
few cones and those that are produced are
smaller than normal and contain fewer
sound seeds.

Other forest values are affected adversely
by fir dwarf mistletoe. Witches’ brooms
and dead branches can increase the hazard
potential in recreation sites because they

Management of fir
dwarf mistletoe must recognize the value
of dwarf mistletoes as functional compo-
nents of forest ecosystems. In areas where
timber production or developed recreation
is the primary goal, direct control of
dwarf mistletoe may be warranted. In
other areas, where wildlife or esthetic val-
ues are more important, maintaining or
even increasing dwarf mistletoe popula-
tions may be appropriate.

Cultural Controls

The only practical control of fir dwarf
mistletoe over large forested areas is
through cultural treatments. No chemical
or biological controls are available for
treating stands. Profitable production of
true fir timber in many local areas often
depends on dwarf mistletoe control.


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-001-00



the spring, the foliage on dead branches
turns reddish-brown, and forms flags that
persist throughout the year.

Bole swellings result from infection of the
main stem of firs. Bole swellings are
more common on the oldest, most severely
infected trees and may break open and
allow decay to develop. Trees may break
at open bole infections and thus pose seri-
ous hazards in developed recreation sites.

Severely infested fir stands typically have
many trees with stunted growth, witches’
brooms, dying or dead tops and branches,
and dead trees. Dieback occurs as nutri-
ents and water needed by growing tree
tops are diverted to infections that are usu-
ally concentrated in the lower or mid
crowns. Foliage near infections becomes
sparse and off-color as they are infected
by canker fungi, and gradually the branch-
es die. Eventually height growth slows,
and such trees may subsequently be
attacked and killed by fir engravers
(Scolytus ventralis LeConte). These
stands eventually contain numerous dying
and dead trees, usually bearing remnants
of dwarf mistletoe infections.

Spread and Intensification

Several interrelated factors influence tree-
to-tree spread of fir dwarf mistletoe.
These include size class, stand structure,
species composition of stands, tree spac-
ing, and infection position. In single sto-
ried stands, spread is estimated to be 1.5 to
2 feet (0.5-0.6 m.) per year. Spread in
multistoried stands is more rapid because
understory trees are bombarded by dwarf
mistletoe seeds from infected overstory
trees.

Presence of non-host tree species can slow
the spread of fir dwarf mistletoe. Spread
rates in very dense stands are less than in
more open stands because dwarf mistletoe
seed production is usually reduced due to
limited light and lower host vigor, and

many seeds are trapped before they travel
far. Dwarf mistletoe seeds from plants
high in crowns tend to travel farther than
those in lower portions of crowns.

Nearly all spread is local and results from
explosive discharge of seeds. Wind exerts
a minor influence on distance and direc-
tion of seed travel. Birds and other ani-
mals are responsible for some long-dis-
tance spread when seeds stick to them and
later are rubbed off onto susceptible trees.

The 6-class dwarf mistletoe rating (DMR)
system is useful for quantifying intensity
of infection in true 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 num-
ber of branches in the crown third have
dwarf mistletoe infections), or 2 = heavy
infection (more than half of the number of
branches in the crown third have infec-
tions). Bole infections are rated as 2.
The three ratings are then added to obtain
a tree rating. The tree ratings of all live
trees (including uninfected ones) are then
averaged to obtain a stand or plot rating.

As a rough rule-of-thumb, intensification
of fir dwarf mistletoe averages about one
DMR class per decade for individual trees,
but varies with tree size, stand position,
and overstory infection. Infection intensi-
fies most rapidly in sapling or pole-size
trees under severely infected larger trees.

Impacts

Infection of fir by dwarf mistletoe causes
increased mortality, reduced growth rates
and loss of vigor, lowered timber quality,
reduced cone and seed production, and
increased susceptibility to other damaging
agents. These damaging effects result
from the dwarf mistletoe plants taking
food and water from the host, thus reduc-
ing the amount available for the tree’s nor-
mal growth, protective, and reproductive


Because the parasite can cause large
reductions in yield, control should be con-
sidered in all timber-producing stands
where dwarf mistletoe occurs.

Successful timber volume production is
almost impossible in multi-storied, infest-
ed fir stands. Silvicultural treatments
designed to achieve single-storied stands
offer the best prospects of preventing
unacceptable losses to dwarf mistletoe.

The most effective method for eliminating
fir dwarf mistletoe from timber-producing
forests is complete harvest of infested
stands by clearcutting. After usable trees
are harvested, all remaining infected trees
should be killed. To minimize invasion of
young fir stands by dwarf mistletoe from
infected border trees, the ratio of perimeter
to area of clearcuts should be as low as
possible. That is, they should be roughly
circular and not long, narrow strips.
Advantage should be taken of any poten-
tial barriers to dwarf mistletoe spread,
such as roads, ridgetops, natural openings,
and changes in timber types when laying
out the boundary of a clearcut.

When clearcutting of dwarf mistletoe-
infested fir stands is not appropriate, shel-
terwood and seed tree harvests can be
good alternative even-age management
methods. Trees selected to provide shelter
or seeds should be uninfected or only
lightly infected (DMR < 3). Moderately
and severely infected trees, in addition to
being a source of dwarf mistletoe seeds,
produce poorer crops of tree seeds.
Infected shelterwood or seed trees should
be removed as soon as susceptible repro-
duction becomes established. As a general
tule for most dwarf mistletoes it is desir-
able to remove the infected overstory
before the young stand is 3 feet (1 m) tall
or 10 years old.

In mixed-species stands that contain fir
infected by dwarf mistletoe, silvicultural
treatments should favor other tree species.
Non-hosts left between infected and non-

Fig. 4. Branch mortality and flags
associated with fir dwarf mistletoe
and cytospora canker in severely
infected grand fir.

infected true firs prevent or slow spread
and intensification of the parasite.

Thinning or sanitation by removal of
infected trees can be an effective treat-
ment in lightly infested stands. Lightly
infested is defined as those stands in
which there are acceptable numbers of
desirable dwarf mistletoe-free or lightly
infected (DMR = | to 2) trees. True firs
with one-half or more of their crowns
infected by dwarf mistletoe (DMR = 3 to
6) may decline rapidly about 10 years
after they are exposed to full sunlight by
thinning. Because of their rapid decline,
moderately infected trees should not be
left when stands are being sanitized unless
they can be expected to reach mer-
chantable size within 15 years.

Thinning priorities of true fir should be
based on both dwarf mistletoe severity
and live crown ratios. First priority for
removal should be all trees with a live
crown ratio < 40% or trees with live crown


gem Forest Insect — Insect
& Disease
Leaflet 89

Revised February 2000

Forest Service

‘US. Department of Agriculture + Forest Service Department of Agriculture .

Fir Dwarf Mistletoe

Gregory M. Filip, Jerome S. Beatty, and Robert L. Mathiasen

Gregory Filip is associate professor,
Department of Forest Science, Oregon
State University, Corvallis, OR; Jerome
Beatty is plant pathologist, USDA Forest
Service, Forest Insects and Diseases,
Pacific Northwest Region, Sandy, OR; and
Robert Mathiasen is assistant professor,
School of Forestry, Northern Arizona
University, Flagstaff, AZ.

Fir dwarf mistletoe (Arceuthobium abiet-
inum Engelmann ex Munz) is a common
and damaging parasite of white fir (Abies
concolor (Gord. & Glend.) Lindl. ex
Hildebr.), grand fir (Abies grandis (Dougl.
ex D. Don) Lindl.), and California red fir
(A. magnifica A. Murr.) in the western
United States.

There are two forms of fir dwarf mistle-
toe: one that attacks white and grand fir
called white fir dwarf mistletoe (4. abiet-
inum Engelmann ex Munz f. sp. concol-
oris Hawksworth & Wiens) and one that
attacks red fir called red fir dwarf mistle-
toe (A. abietinum Engelmann ex Munz f.
sp. magnificae Hawksworth & Wiens).

White fir dwarf mistletoe occurs on grand
and white fir in the Cascade Mountains of
southern Washington, Oregon, and north-
ern California, the Siskiyou Mountains of
southern Oregon and northern California,
and in the Sierra Nevada Mountains in
California. It has also been reported on
white fir in southern Nevada and Utah, in

a few locations in Arizona, and on
Durango fir (Abies durangensis Matt.)
from two locations in Chihuahua, Mexico
(Figure 1 - map of white fir dwarf mistle-
toe distribution).

Red fir dwarf mistletoe occurs on
California red fir and Shasta red fir (A.
magnifica var. shastensis Lemmon), in the
Cascade Mountains of Oregon and north-
ern California, the Siskiyou Mountains of
southern Oregon and northern California,
and in the Sierra Nevada Mountains in
California (Figure 2 - map of red fir dwarf
mistletoe distribution).

Fir dwarf mistletoe is common on true firs
in the Cascade and Sierra Nevada
Mountains. For example, surveys of
national forests in Oregon, Washington,
and California indicated that over 20 per-
cent of the true fir forests in this region
were infested with fir dwarf mistletoe.

Fir dwarf mistletoe parasitizes several tree
species in several different genera. White,
grand, and red fir are the principal tree
species affected by fir dwarf mistletoe.
Brewer spruce (Picea breweriana Wats.)
is severely parasitized in the Siskiyou
Mountains of southern Oregon when asso-
ciated with infected white fir. Subalpine
fir (Abies lasiocarpa (Hook.) Nutt is occa-
sionally parasitized when growing with
infected white fir in northern Arizona and
infected red fir in northern California.


ratios < 60% with a DMR of 5 or 6. Last
trees to be removed should be those with a
live crown ratio > 80% and DMR < 3.

In stands where management of hiding and
thermal cover for wildlife is a primary
consideration, infected trees with large
witches’ broom should be treated by
girdling rather than felling. Reexami-
nation of stands 5 years after sanitation is
desirable to determine if additional sanita-
tion is needed.

Moderately and severely infested stands
should not be sanitized because many trees
with latent infections will be missed, and
once these trees are released from compe-
tition, the number of dwarf mistletoe
plants will increase rapidly within their
crowns. Such stands should either be
clearcut and 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.

Recreation Management

In recreation areas, sanitation treatments
that favor non-susceptible hosts or remove
infected overstory trees are appropriate.
Pruning, in conjunction with removal of
severely infected trees, can prolong the life
of individual trees. For high value trees,
removal of live witches’ brooms that exert
a large drain on infected trees can improve
their vigor and prolong their life. Trees
may have to be repruned occasionally to
remove developing witches’ brooms.
Candidates for pruning are trees with
infections only in the lower half of their
crown. In addition to prolonging tree life,
pruning can reduce the danger of trees or
branches breaking and causing accidents.
Trees with bole infections may contain

decayed wood and therefore have a high
potential to fail.

Assistance

Resources managers and forest landowners
can get more information about the identi-
fication and management of fir dwarf
mistletoe by contacting a County
Cooperative Extension Agent, their state
forestry office, or their local USDA Forest
Service, Forest Health Protection office.

References

Filip, Gregory M. Dwarf mistletoe and
Cytospora canker decrease grand fir
growth in central Oregon. Forest Science
30:1071-1079. 1984

Hawksworth, Frank G. The 6-class dwarf
mistletoe rating system. Research Note
RM-48. Rocky Mountain Forest and
Range Experiment Station, Fort Collins,
CO: U.S. Department of Agriculture,
Forest Service; 1977. 7 p.

Hawksworth, Frank G.; Wiens, Delbert.
Dwarf mistletoes: Biology, pathology,
and systematics. Agric. Handb. 709.
Washington, D. C.: U.S. Department of
Agriculture, Forest Service; 1996. 410 p.

Parmeter, John R., Jr.; Scharpf, Robert F.
Stem infection by dwarf mistletoe in
California true firs. Research Paper PSW-
165. Pacific Southwest Forest and Range
Experiment Station, Berkeley, CA: U.S.
Department of Agriculture, Forest Service;
1983. 7p.

Scharpf, Robert F. Cytospora abietis
associated with dwarf mistletoe on true
firs in California. Phytopathology
59:1657-1658. 1969

Scharpf, Robert F.; Parmeter, John R., Jr.
Population dynamics of dwarf mistletoe on
young true firs in central Sierra Nevada,
California. Research Paper PSW-161.
Pacific Southwest Forest and Range
Experiment Station, Berkeley, CA: U.S.
Department of Agriculture, Forest Service;
1982. 9 p.


Pacific silver fir (Abies amabilis (Dougl.)
Forbes), western white pine (Pinus monti-
cola Dougl. ex D. Don), sugar pine (P.
lambertiana Dougl.), and lodgepole pine
(P. contorta var. murrayana (Grev. &
Balf.) Engelm.) are rare hosts of this
mistletoe.

In the Cascade Mountains of central
Oregon, Pacific silver fir and noble fir
(Abies procera Rehd.) are severely para-
sitized by mountain hemlock dwarf mistle-
toe (Arceuthobium tsugense (Rosendahl)
G.N. Jones ssp. mertensianae Hawksw. &
Nickrent), but not by the fir dwarf mistle-
toe. Mountain hemlock dwarf mistletoe
plants on true firs are similar in size to

those of fir dwarf mistletoe, but mountain
hemlock dwarf mistletoe plants are gener-
ally greenish or reddish in color and this
dwarf mistletoe severely parasitizes moun-
tain hemlock (7suga mertensiana (Bong.)
Carr.) when this host is associated with
infected true firs. Fir dwarf mistletoe has
not been reported to parasitize mountain
hemlock, thus far.

Life History

Fir dwarf mistletoe is a small, seed-bear-
ing parasitic plant. The external (aerial)
shoots are yellow-green to yellow, leaf-
less, and perennial. Their average height
is only about 3 inches (8 cm), but they are

sometimes as tall as 9 inches (22
cm) (Figure 3).

Aerial shoots arise from a net-
work of root-like absorbing
strands imbedded in host tissues.
This network, called the endo-
phytic system, consists of cortical
strands growing within the bark
and sinkers within the wood.
The endophytic system obtains
nutrients and water from the host
tree. The endophytic system
lives as long as adjacent host tis-
sues are alive.

The mistletoe is dependent upon
its host trees for water and nutri-
ents, although the aerial shoots
contain chlorophyll that allows
them to produce small amounts
of carbohydrates. The major
function of aerial shoots is repro-
duction. Male and female flow-
ers are small and produced on
separate plants. Flowering takes
place in August and September.
Insects and wind are both
involved in pollination.

Fig. 1. Distribution of white fir dwarf
mistletoe in the western United States.

Fruits complete their develop
ment 13 to 14 months after polli-
nation. Mature fruit contain one


Fig. 2. Distribution of red fir dwarf
mistletoe in the western United States.

seed averaging about 0.1 inch (3 mm.) in
length. Seed dispersal is one of the most
interesting characteristics of dwarf mistle-
toes. Seeds are discharged explosively
from ripe fruits in September and October.
They may travel 30 to 40 feet (10-13 m.),
but most land within 10-15 feet (3-5 m.)
of the disseminating shoot. A sticky
hygroscopic seed coating called “viscin”
enables seeds to stick to most objects they
strike.

Foliage is the most common receiving sur-
face. Viscin, when first moistened by
rains, acts as a lubricant. Seeds slide
down and either fall off needles or become
lodged on bark at the base of needles.
Seeds are fastened in place when the
viscin dries and they overwinter in a dor-
mant state. Seeds are often destroyed by
insects and fungi or dislodged by rains and
snow, so only a small proportion of the

seeds dispersed actually survive and give
rise to new plants.

Seeds germinate in early spring. Radicles
grow along the bark surface until an
obstruction, usually a needle base, is
encountered. The radicles then form
mounds of tissue called holdfasts. Host
tissue is then penetrated by infection pegs
that develop from the holdfasts during the
summer. The mistletoe’s endophytic sys-
tem then develops in the cortex and wood
of the host. Infection occurs most readily
in 1-year old twigs because their bark is
more easily penetrated than older twigs.

Aerial shoots typically appear 2 to 3 years
after initial infection. 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 dissemination of the first seed crop is 4
to 5 years. Many successive crops of aeri-
al shoots may be produced from the estab-
lished endophytic system.

Symptoms and Signs of Infection

The first symptom of dwarf mistletoe
infection is the appearance of slight
swellings at infection sites. Swellings
become visible 1 to 2 years after infection
occurs. Witches’ brooms occasionally are
associated with old infections. Witches’
brooms are variously shaped masses of
abnormal branch and twig growth. Dwarf
mistletoe brooms are readily distinguished
from the tight, ball-shaped, brooms caused.
by the rust fungus, Melampsorella
caryophyllacearum Schroet., that infects
true firs.

Often the most conspicuous symptom of
fir dwarf mistletoe are flags (Figure 4).
Flags are dead branches that are caused by
canker-forming fungi, most commonly,
Cytospora abietis Sacc., that invade dwarf
mistletoe swellings and kill the host
branches during the dormant season. In