Forest Insect 

and Disease 

Leaflet 

NA-PR-04-06 

Revised 2006 

U.S. Department of Agriculture • Forest Service 

EASTERN SPRUCE DWARF MISTLETOE 

F. A. Baker, J.G. O’Brien, Robert Mathiasen, and M.E. Ostry

Description 

Eastern spruce dwarf mistletoe 

(Arceuthobium pusillum) is a parasitic flowering plant that causes the 

most serious disease of black spruce 

(Picea mariana) throughout its range. 

The parasite occurs in the Canadian 

provinces of Saskatchewan, Manitoba, Ontario, Quebec, New Brunswick, Nova Scotia, Prince Edward 

Island, and Newfoundland; in the 

Lake States of Minnesota, Wisconsin, 

and Michigan; and the Northeastern 

states of New York, Pennsylvania, 

Vermont, New Hampshire, and 

Maine. (Figure ). The eastern 

spruce dwarf mistletoe is rarely 

found in Rhode Island, Connecticut, 

New Jersey, and Massachusetts. 

Arceuthobium pusillum is most 

damaging in black spruce stands. 

Black spruce is a valuable species 

used in the manufacture of highFigure 1. Distribution of Arceuthobium 

pusillum (from Hawksworth and Wiens 

1996). 

quality paper. Primarily a lowland 

species, black spruce is often the only 

commercially important species that 

can grow on those sites. Therefore, it 

is important to protect black spruce 

from dwarf mistletoe infection. In an 

aerial survey of important spruceproducing forests in northern Minnesota, Anderson (99) conservatively estimated that -% of the 

land was out of production due to 

 Fred Baker is Professor, Utah State University, Logan, UT.  Joseph O’Brien, Plant Pathologist, USDA Forest Service, Forest Health Protection,

St. Paul, MN.  Robert Mathiasen, Associate Professor, School of Forestry, Northern Arizona University, 

Flagstaff, AZ.  M.E. Ostry, Plant Pathologist, USDA Forest Service, North Central Research Station,

St. Paul, MN. 
eastern spruce dwarf mistletoe. 

Although white spruce (Picea 

glauca) and red spruce (Picea 

rubens) are also highly susceptible to 

this parasite, eastern dwarf mistletoe 

is not as common on these trees, 

perhaps because they rarely occur in 

pure stands. In the United States 

white spruce is frequently infected 

along the coast of Maine and on the 

north shore of Lake Michigan. Red 

spruce is commonly infected in old 

growth forests in New York, Vermont, 

New Hampshire, and Maine. Other 

hosts include eastern larch (Larix 

laricina), jack pine (Pinus banksiana), eastern white pine (Pinus 

strobus), red pine (Pinus resinosa), 

balsam fir (Abies balsamea), and blue 

spruce (Picea pungens), but these 

hosts are infected only when growing 

near other infected species of spruce. 

Life History 

Dwarf mistletoes are small, seedbearing, parasitic plants. Eastern 

dwarf mistletoe has green to brown 

external (aerial) shoots, usually no 

secondary branching, and leaves 

reduced in size to small scales. The 

shoots are perennial, usually about 

0. -.0 inches (-.5 cm) long. The 

major function of aerial shoots is reproduction. Male and female flowers 

are small and produced on separate 

plants (Figure ). Flowering takes 

place from late March until early 

June, peaking in April and May. Insects and wind are involved in pollination of female flowers. Male aerial 

shoots are shed soon after flowering, 

female shoots are shed after they disperse their seeds. Basal cups remain 

Figure 2. Flowers on staminate (male; 

top) and pistillate (female; bottom) 

Figure 3. Basal cups remaining after 

aerial shoots are shed. 

on the portions of branches where 

aerial shoots had formed (Figure ). 

Dwarf mistletoe plants contain 

chlorophyll so they produce some 

photosynthate, but they obtain most 

of their nutrients from the living 

tissues of the host through what is 

called the endophytic system. This 

root-like network consists of cortical strands growing within the bark 

and sinkers within the wood. The 

 
Figure 4. Mature fruits (berries) on a 

pistillate plant. 

endophytic system lives as long as 

adjacent host tissues are alive. 

Fruits mature in August or September 

of the same year they were pollinated. Each mature fruit contains one 

seed about 0. inch ( mm) in length 

(Figure ). Seeds are discharged 

explosively from ripe fruits in August 

and September. They may travel 

as far as 55 feet (6.5 m), but most 

land within 0-5 feet (-5 m) of the 

disseminating shoot. A sticky seed 

coating called viscin enables seeds to 

stick to objects they strike. Foliage is 

the most common receiving surface. 

Seeds also can stick to the bodies 

of birds and squirrels as they forage 

in infected trees. Animals can inadvertently carry seeds to trees farther 

Figure 5. Dwarf mistletoe seed attached 

to black spruce needles with viscin 

threads. 

away than by natural discharge, 

potentially starting new infection 

centers. 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 (Figure 5). Seeds are fastened in place when the viscin dries 

and they overwinter in a dormant 

state. Many seeds are destroyed by 

insects and fungi or dislodged by rain 

and snow, so only a small proportion of the those dispersed actually 

survive and give rise to new plants. 

Figure 6. Germinating seed of Arceuthobium pusillum with its characteristic red 

radicle (arrow). 

Seeds germinate in the spring. A 

structure called a radicle emerges 

from a germinating seed and grows 

along the bark surface, and penetrates 

the host tissue (Figure 6). The 

mistletoe’s endophytic system then 

develops in the bark and wood of the 

host. Infection occurs most readily 

in - to 5-year- old twigs because 

their bark is more easily penetrated 

than older twigs. For two or more 

years, these infections are quiescent, 

or latent, and there are no symptoms. 

The first symptom is a swelling at the 

point of infection. Buds proliferate 

 
Figure 7. Epinastic effect in infected 

tissue (left image). Apical dominance is 

disrupted. 

Figure 8. Dwarf mistletoe infected 

branches have longer internodes (left 

image) than uninfected tissue in vigorous 

trees. 

Figure 9. Developing witches’ broom. 

Note the orange discoloration of the bark 

of infected tissue. 

at this point, giving rise to a witches’ 

broom, a compact mass of branches 

and twigs. The apical dominance of 

infected tissue is released, and the 

branches often exhibit an epinastic 

effect (Figure 7). If the tree is vigorous, the infected tissue may have 

longer internodes than uninfected 

tissue (Figure 8). Initial growth of 

the witches’ broom may be quite 

vigorous (Figure 9). Aerial shoots 

typically appear  years after infection, and these produce flowers and 

fruits in their fifth year. Thus, plants 

need at least 5 years to complete 

their life cycle from initial establishment to dissemination of the first 

seed crop. Many successive crops of 

aerial shoots may be produced from 

the established endophytic system. 

Symptoms and Signs of 

Infection 

Witches’ brooms are the most apparent symptom of dwarf mistletoe 

infection on spruce (Figure 0). 

They live as long as the host remains 

Figure 10. Large witches’ brooms on infected trees. Note the thin upper crowns 

of these declining trees. 

 
alive and may reach -0 feet (- 

m) in diameter. Branches in these 

brooms may have several mistletoe 

plants on them. Uninfected tissues 

decline first, until nearly all the 

foliage is contained in the witches’ 

brooms, and the tree is near death. 

The upper crown often dies first, 

so severely infected trees usually 

have dead tops. After a tree dies 

and loses its needles, basal cups 

on branches allow observers to 

distinguish mistletoe brooms from 

those associated with spruce broom 

rust (Chrysomyxa arctostaphyli). 

Spread and Intensification 

While many factors influence tree-totree spread of eastern spruce dwarf 

mistletoe, these are not terribly important because the trees are killed so 

quickly. The parasite spreads about 

. feet per year, as measured on the 

ground between the boles of infected 

trees. The spread rate through a 

stand, as indicated by the enlargement of mortality centers, is almost 

double this at .7 feet per year. While 

we might expect spread rates in very 

dense stands to be less than in more 

open stands, the rapid mortality 

of infected trees negates any such 

effects. Nearly all spread is local and 

results from explosive discharge of 

seeds. Wind exerts a minor influence 

on distance and direction of seed 

travel. Birds and other animals with 

seeds on their bodies can rub them 

off onto susceptible trees, creating 

new mortality centers. The 6-class 

dwarf mistletoe rating (DMR) system 

in common use in the western U.S. 

is not useful for quantifying intensity of infection in stands of black 

and white spruce. Dwarf mistletoe 

kills these trees so quickly, often 

within 5 years, that there is little 

benefit to rating disease intensity. 

Impacts 

Damage to spruce due to mistletoe 

infection includes 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 reducing the amount available 

for the tree’s normal growth, defense, 

and reproduction. Dwarf mistletoe is 

the major cause of reduced stocking 

in the black spruce forest type. In severely infested areas, stocking levels 

are so low that a commercial harvest 

is impossible. Dwarf mistletoe kills 

young spruce saplings, which also 

contributes to reduced stocking. 

Weakened trees are more susceptible 

to drought and attack by insects 

and fungi. Black spruce growing 

on organic sites are not windfirm. 

Where the stand canopy has been 

opened by mortality caused by 

dwarf mistletoe, large losses 

due to windthrow can occur. 

Dwarf mistletoes are a functional 

component of many forest ecosystems, having intrinsic value as 

individual species and as disturbance 

agents, influencing both the structure 

and function of forest communities. 

In areas where wildlife and plant 

diversity or aesthetic values are more 

important, allowing a dwarf mistletoe 

5 
infestation to continue may be appropriate. In other areas where timber 

production or developed recreation 

is the primary goal, direct control of 

dwarf mistletoes may be warranted. 

Management 

Effective management of mistletoeinfested black spruce stands requires 

eradication of the parasite because 

A. pusillum kills black spruce very 

quickly, creating mortality centers 

in a stand. The mortality center 

continues to enlarge during rotation, 

ultimately removing significant 

areas of the stand from production. 

In stands managed for timber, the 

mortality caused by dwarf mistletoe 

is almost always unacceptable. To 

prevent these losses, the mistletoe 

should be eradicated from regenerating stands. Volume losses and area 

out of production can be projected 

using DMLOSS, which has recently 

been modified to work with a geographic information system (Table 

). Any treatment that kills all trees 

surviving on the site, infected or 

not (because trees may be infected 

without showing symptoms), will 

eliminate dwarf mistletoe. Prescribed 

burning the slash remaining after 

logging can eliminate residual black 

spruce. In addition to consuming 

or scorching residual trees, fire also 

favors sphagnum mosses, which 

provide an optimal seed bed for black 

spruce. However, black spruce stand 

are commonly harvested using full 

tree and tree length logging systems. 

These logging methods rarely yield 

the aerial slash necessary to carry a 

fire of sufficient intensity to kill residual trees. Harvesting equipment, if 

not restricted by deep snow, shallow 

frost, or both, can kill the residuals, 

and eliminate dwarf mistletoe in the 

regenerating stand. Where residual 

trees survive harvesting, additional 

measures are required. Dispatching 

crews to cut residuals with chain 

saws or brush saws has not been 

effective, perhaps because only infected trees were cut, and “apparently 

mistletoe-free” trees were infected. 

Sodium trichloroacetate (TCA) can 

effectively kill black spruce at a 

reasonable cost, without affecting the 

Table 1. DMLOSS projection of losses in a 15-hectare infested black spruce stand. 

This stand regenerated with 16 infected trees per acre. Treatment area includes a 

20-m buffer around the infested area. 

Stand Age 

Infested 

area (ha) 

Stand 

volume 

without 

dwarf 

mistletoe 

(m3) 

Stand 

volume 

with dwarf 

mistletoe 

(m3) 

Stand 

volume 

lost (m3) 

Volume 

lost (%) 

Treatment 

area (ha) 

32 0.3 3.1 

42 1.0 1,160 1,127 33 3 4.0 

52 1.9 1,247 1,157 91 7 6.1 

62 3.1 1,321 1,140 181 14 7.5 

72 4.4 1,396 1,090 305 22 8.8 

82 5.7 1,466 1,019 447 30 9.9 

6 
sphagnum moss seedbed or germinating spruce seeds. Regardless of the 

method, all residual black spruce 

must be killed to ensure that the 

regenerating stand is free of dwarf 

mistletoe. Mechanical treatments 

such as shearing or using a hydroaxe 

may be used to eliminate residual 

spruce, but cost, access, and ability 

to move on the site during most of 

the year can limit their effectiveness. 

Leaving even 6 infected trees per 

acre after harvest has allowed dwarf 

mistletoe to increase and cause 

serious losses in the regenerated 

stand (Table ). Regardless of how 

residuals on the site are eliminated, 

treatment block boundaries must be 

established in mistletoe-free areas. 

Harvesting should extend at least  

chains (0 m) beyond obvious infection to ensure that all latent infections 

are removed. If a dwarf mistletoefree perimeter cannot be established 

in regenerating stands, an area 

0-0 m wide adjacent to the infestation should be maintained free of host 

plants or regenerated with a less susceptible species such as eastern larch. 

A buffer of resistant species will 

provide a barrier to dwarf mistletoe 

invasion of the regenerating stand. 

Assistance 

Resource managers and forest 

landowners can get more information about the identification and 

management of black spruce dwarf 

mistletoe by contacting their local 

state forestry office, or their regional USDA Forest Service, Forest 

Health Protection (FHP) office. 

References 

Anderson, R.L. 99. The spruce 

dwarf mistletoe in Minnesota., University of Minnesota, 

St. Paul, 9p. Masters Thesis. 

Baker, F.A. 98. Biology and control 

of the eastern dwarf mistletoe. 

PhD Thesis, University of Minnesota, Minneapolis, MN 5p. 

Baker, F.A.; French, D.W. 980. 

Spread of Arceuthobium pusillum and rates of infection and 

mortality in black spruce stands. 

Plant Disease. 6: 07-076. 

Baker, F.A.; French, D.W. 99. Radial 

enlargement of mortality centers 

caused by Arceuthobium pusillum Peck in black spruce stands. 

Forest Science. 7: 6-67.

 Baker, F.A.; French, D.W.; Hudler, 

G.W. 98. Development of 


Arceuthobium pusillum on 


inoculated black spruce. For­


est Science. 7: 0-05.


Baker, F.A.; French, D.W.; Rose, 

D.W. 98. DMLOSS: a simulator of losses in dwarf mistletoe 

infested black spruce stands. 

Forest Science. 8: 590-598. 

Baker, F.A.; Knowles, K. 00. Case 

study: 6 years of dwarf mistletoe 

in a regenerating black spruce 

stand. Northern Journal of Applied Forestry. : 50-5. 

Hawksworth, Frank G.; Wiens, 

D. 996. Dwarf mistletoes: 

biology, pathology, and systematics. USDA Forest Service, 

Agric. Handbk. 709 0p. 

Ostry, M. E.; Nicholls, T. H. 976. How 

to identify eastern dwarf mistletoe 

on black spruce. USDA Forest 

Service, North Central Forest Experiment Station, St. Paul, MN. 5 p. 

7 
Ostry, M.E.; Nicholls, T.H.; French, 

D.W. 98. Animal vectors of eastern spruce dwarf mistletoe of black 

spruce. Res. Pap. NC-. St. Paul, 

MN: U.S. Department of Agriculture, Forest Service, North Central 

Forest Experiment Station.6p. 

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