Forest Insect and Disease Leaflet Number 158

Revised 2003


F. A. Baker, Joe O'Brien,

and Robert Mathiasen

Fred Baker is Professor, Utah State University, Logan, UT; Joe O'Brien is Plant Pathologist, USDA Forest Service, Forest Health Protection, St. Paul, MN; and Robert Mathiasen is Associate Professor, School of Forestry, Northern Arizona University, Flagstaff, AZ; MEO??*

Eastern 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 Saskatchewan, Manitoba, Ontario, Quebec, New Brunswick, Nova Scotia, Prince Edward Island, and Newfoundland, in the Lake States of Minnesota, Wisconsin, and Michigan, and New York, Pennsylvania, Vermont, New Hampshire, and Maine. The eastern dwarf mistletoe is rarely found in Rhode Island, Connecticut, New Jersey, and Massachusetts.

The parasite is most damaging in black spruce stands. Black spruce is a valuable species,

Figure 1. Distribution of Arceuthobium pusillum (from Hawksworth and Wiens 1996).

used in the manufacture of high-quality paper. Primarily a lowland species, it 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 spruce producing forests in northern Minnesota, Anderson (1949) conservatively estimated that 3-11% of the land was out of production due to eastern 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, seed-bearing parasitic plants. The external (aerial) shoots of eastern spruce dwarf mistletoe are green to brown, usually don't have secondary branching, and have their leaves reduced in size to small scales. The shoots are perennial and very small, usually about 0.4 -1.0 inches (1-2.5 cm) long. Dwarf mistletoe plants contain chlorophyll so they produce some photosynthate, but most of their nutrients are obtained 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 endophytic system lives as long as adjacent host tissues are alive.

The major function of aerial shoots is reproduction. Male and female flowers are small and produced on separate plants (Figure 2). Flowering takes place from late March until early June, with peak flowering in April and May. Insects and wind are both 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 on the portions of branches where aerial shoots had formed. (Figure 3).

Figure 2. Flowers on staminate (male) plants.   Figure 3. Basal cups remaining after aerial shoots are shed.

Fruits mature in August or September of the same year they were pollinated. Mature fruit contain one seed about 0.1 inch (3 mm.) in length (Figure 4). Seed dispersal is one of the most interesting characteristics of dwarf mistletoes. Seeds are discharged explosively from ripe fruits in August and September. They may travel 55 feet (16.5 m.), but most land within 10-15 feet (3-5 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. 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 over winter in a dormant 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.

Figure 4. Mature fruits (berries) on a pistillate plant.   Figure 5. Germinating seed of Arceuthobium pusillum with its characteristic red radicle.

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 5). The mistletoe's endophytic system then develops in the bark and wood of the host. Infection occurs most readily in 1- to 5-year old twigs because their bark is more easily penetrated than older twigs. For two or more years, there are no symptoms of infection; these infections are quiescent, or latent. The first symptom is a swelling at the point of infection. Buds proliferate at this point, giving rise to a witches' broom. The apical dominance of infected tissue is released, and the branches often exhibit an epinastic effect ( Figure 6). If the tree is vigorous, the infected tissue may have longer internodes than uninfected tissue (Figure 7). Initial growth of the witches' broom may be quite vigorous (Figure 8). Aerial shoots typically appear 4 years after infection, and these produce flowers and fruits in their 5th year. Thus, the minimum time needed for female plants to complete their life cycle from initial establishment to dissemination of the first seed crop is 5 years. Many successive crops of aerial shoots may be produced from the established endophytic system.

Figure 6. Epinastic effect in infected tissue; apical dominance is disrupted.   Figure 7. Dwarf mistletoe infected tissue grows longer than uninfected tissue in vigorous trees.
Figure 8. Developing witches' broom. Note the orange discoloration of the bark of infected tissue.   Figure 9. Large witches' brooms on infected trees. Note the thin upper crowns of these declining trees.

Symptoms and Signs of Infection

The most apparent symptom of dwarf mistletoe infection on spruce are compact masses of branches and twigs called witches' brooms (Figure 9). These live as long as the host remains alive and may reach 3-10 feet (1-3 m) in diameter. Branches in these brooms may have several mistletoe plants on them. Uninfected tissues decline first, until nearly all of 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 distinguishing mistletoe brooms from those associated with spruce broom rust (Chrysomyxa arctostaphylii), which also has deciduous foliage.

Spread and Intensification

While many factors influence tree-to-tree spread of eastern spruce dwarf mistletoe, these are not terribly important, because the trees are killed so quickly. The parasite spreads about 2.4 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 4.7 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 can carry seeds and rub them off onto susceptible trees, but such occurrences occur relatively rarely and are not important in timber management.

The 6-class dwarf mistletoe rating (DMR) system is not useful for quantifying intensity of infection in stands of black and white spruce. Dwarf mistletoe kills these trees so quickly, often within 15 years, that there is little benefit to rating disease intensity.


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, protective, and reproductive processes. 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.

In forest ecosystems, dwarf mistletoes have value as individual, biological species and act as disturbance agents, influencing both the structure and function of forest communities. Management of dwarf mistletoes must recognize their value as functional components of forest ecosystems. In areas where timber production or developed recreation is the primary goal, direct control of dwarf mistletoes may be warranted. In other areas, where wildlife or esthetic values are more important, allowing the infestation to continue may be appropriate.


Unlike the species of dwarf mistletoes that occur in the western US and Canada, effective management of mistletoe-infested 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 1).

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 and/or shallow frost, can kill the residuals, and eliminate the establishment of dwarf mistletoe in the regenerating stand. Where residuals 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 TCA can effectively kill black spruce at a reasonable cost. Regardless of the method, all residual black spruce must be killed to ensure that the regenerating stand is free of dwarf mistletoe. Leaving even 16 infected trees per acre after harvest has allowed dwarf mistletoe to increase and cause serious losses in the regenerated stand (Table 1).

Regardless of how residuals on the site are eliminated, treatment block boundaries must be established in mistletoe-free areas. Harvesting should extend at least 2 chains (40m) beyond obvious infection to ensure that all latent infections are removed. If a dwarf mistletoe-free perimeter cannot be established, an area adjacent to the infestation 20-40m wide should be maintained free of host plants or regenerated with a non-host species such as eastern larch. This non-hose buffer will provide a barrier to dwarf mistletoe invasion of the regenerating stand.


Resources 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.

Table 1. DMLOSS projection of losses in a 15-hectare infested black spruce stand. This stand regenerated with 1 infected trees per acre.

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 1160 1127 33 3 4
52 1.9 1247 1157 91 7 6.1
62 3.1 1321 1140 181 14 7.5
72 4.4 1396 1090 305 22 8.8
82 5.7 1466 1019 447 30 9.9



Anderson, R.L. 1949. The spruce dwarf mistletoe in Minnesota. MS Thesis, University of Minnesota, St. Paul, 139p.

Baker, F.A. 1981. Biology and control of the eastern dwarf mistletoe. PhD Thesis, University of Minnesota, Minneapolis, MN 254p.

Baker, F.A. and French, D.W. 1980. Spread of Arceuthobium pusillum and rates of infection and mortality in black spruce stands. Plant Disease 64:1074-1076.

Baker, F.A., and French, D.W. 1991. Radial enlargement of mortality centers caused by Arceuthobium pusillum Peck in black spruce stands. For. Sci. 37:364-367.

Baker, F.A., French, D.W., and Hudler, G.W. 1981. Development of Arceuthobium pusillum on inoculated black spruce. Forest Sci. 27:203-205.

Baker, F.A., French, D.W., and Rose, D.W. 1982. DMLOSS: a simulator of losses in dwarf mistletoe infested black spruce stands. Forest Sci. 28:590-598.

Baker, F.A., and Knowles, K. 2003. Case study: 36 years of dwarf mistletoe in a regenerating black spruce stand. Submitted to Northern Journal of Applied Forestry.

Hawksworth, Frank G., and Wiens, D.. 1996. Dwarf mistletoes: biology, pathology, and systematics. USDA Forest Service, Agriculture Handbook 709, 410 p

Ostry, M. E., and Nicholls, T. H. . 1976. How to identify eastern dwarf mistletoe on black spruce. USDA Forest Service, North Central Forest Experiment Station, St. Paul, MN, 5 p.