Eastern dwarf mistletoe

Written by A.C. Skinner Revision 2026

• French disease name: Faux-gui de l'est
• Other disease names: dwarf mistletoe, witches broom
• Pathogen name: Arceuthobium pusillum Peck
• Kingdom: Plantae
• Phylum: Tracheophyta
• Class: Magnoliopsida
• Order: Santalales
• Family: Viscaceae
• Partial list of synonyms:
  • Razoumofskya pusilla (Peck) Kuntze 

General information and importance

The eastern dwarf mistletoe (Arceuthobium pusillum) is a tiny parasitic plant that primarily infects branches of spruce. Despite having the smallest shoots of any North American dwarf mistletoe, it is a significant pathogen that extracts the bulk of its nourishment from its host. Dwarf mistletoes rob resources such as assimilated carbon, water, and mineral nutrients from their hosts that could otherwise be used by the host tree for growth and reproduction. In Canada, the majority of economic damage from this pathogen occurs in forest stands that are valuable to the pulp and paper industry and lumber production. Trees heavily infected with dwarf mistletoe often exhibit deformity and stunted growth, substantially reducing the quantity and quality of commercial timber.

Distribution and hosts

Eastern dwarf mistletoe (A. pusillum) has a range throughout eastern Canada and the eastern United States. This native parasitic plant has been documented in Saskatchewan, Manitoba, Ontario, Quebec, the Maritime Provinces, and Newfoundland and Labrador. In the United States, eastern dwarf mistletoe has been found in Minnesota, Wisconsin, Michigan, New York, Pennsylvania, Vermont, New Hampshire, and Maine. There are 7 other species of dwarf mistletoe in North America on various conifers. A. pusillum is the only species of dwarf mistletoe found in eastern North America.

Black spruce (Picea mariana) is the preferred host, with this mistletoe commonly found on it throughout most of its eastern range. Additionally, A. pusillum infects white spruce (P. glauca), red spruce (P. rubens), eastern larch (Larix laricina), and eastern white pine (Pinus strobus). Jack pine (P. banksiana), red pine (P. resinosa), balsam fir (Abies balsamea), and blue spruce (Picea pungens) serve as rare hosts, typically only when situated near other infected spruce species.

Tree parts affected

Twigs, branches, and occasionally stems

Symptoms and signs

Eastern dwarf mistletoe infections stimulate excessive branching of their host trees, leading to abnormal, bushy growths known as witches’ brooms. The fungal plant disease yellow witches’ broom (Chrysomyxa arctostaphyli) also causes similar brooms on spruce, potentially leading to confusion with dwarf mistletoe infections. The distinguishing features of yellow witches’ broom include the complete loss of needles in winter and the yellow-orange colour of the broom growths in spring and summer.

Two types of infections exist:

•     Localized infections, which result in spindle-shaped swellings of twigs or branches at the points of infection;        and

             •   Systemic infections, which cause brooms

Localized Arceuthobium pusillum infections typically produce stems ranging in colour from green to brown, and occasionally purplish. Stems are circular when fresh and 4-sided when dry. Eastern dwarf mistletoe plants, similar in size to spruce needles, are easily overlooked because of their tiny size and relatively inconspicuous nature. Shoot height averages 1 centimetre, often without secondary branching. The opposite leaves of the parasitic plant are fused around the stem where they meet, have a width of 1 millimetre, look like blunt-tipped scales, and may be mistaken for spruce buds by someone unfamiliar with both the host and the parasite.

The eastern dwarf mistletoe is dioecious, having both male and female plants that are located on separate branches of a single tree or on separate trees. The male, pollen-producing flowers, which survive only a short time, are 1.8 millimetres wide and orange-yellow. Male flowers have 3 sepals each with a tiny yellow anther (pollen) sac on its upper surface. Female flowers have 3 or 4 floral leaf segments that are 0.8 millimetres × 0.7 millimetres. Both male and female flowers exude nectar and are primarily pollinated by insects but wind, birds, and animals likely play some role as well.

Each female flower, once pollinated, results in 1 barrel-shaped green fruit, measuring 2 millimetres × 0.9 millimetres. When seeds are mature, they are expelled explosively from fruits. The hard seeds can travel up to 15 metres. Remarkably, when scaled to comparable size, this trajectory exceeds that of World War II battleship guns! Seeds are covered with a viscous layer that serves as a lubricant when wet, allowing them to slide down needles, and as an adhesive when dry, helping them to stick to whatever surface they land on.

Disease cycle

Dispersal of eastern dwarf mistletoe seeds begins in late August or September, when mature fruit explosively discharge their seeds into ballistic flight. The sticky seeds are also fortuitously carried by birds and mammals, providing additional dispersal for the species. The adhesive-coated seeds adhere to needles and when moistened by rain or dew, slide down due to gravity and germinate on live host branches. Seeds that have landed on needles on the underside of branches will likely slide off to land elsewhere on the host or on the ground. Seeds germinate the following spring after dispersal. Germinating seeds penetrate through the bark with a root-like structure known as a holdfast to connect to internal host tissues. Over the next several years, the dwarf mistletoe will form extensive cellular connections within the host, depleting it of water, nutrients, and carbohydrates through its endophytic system.

In late summer or autumn, 2 to 5 years after initial penetration and successful infection, aerial shoots emerge from tree branches near the infection site. These shoots first appear as small eruptions in the bark of host branches and mature the following season. Although dwarf mistletoe shoots contain chlorophyll, they have minimal photosynthetic significance. Their primary function is reproductive, with secondary roles in water regulation and the synthesis of growth compounds.

Tiny flowers develop in the spring after shoot maturity, typically opening from April to May. Following pollination, it takes about 5 months for the fruits to mature, usually by late August or September. Eastern dwarf mistletoe shoots die after flowering, leaving only a cup-shaped base in the bark. The plant rarely produces a second crop of flowers from the same shoots. The parasite generally survives unless the host tree dies.

Damage

This parasitic plant infects trees of all ages, causing reduced growth, deformities, and sometimes death. Trees infected with eastern dwarf mistletoe have smaller, yellowish-green needles compared with healthy trees. Younger trees are killed more rapidly by the parasitic plant compared with older trees. Infected trees also produce seeds with reduced viability and quality. One incidental benefit of infection is that broom clusters provide cover and nesting sites for many different birds and mammals.

Localized infections can result in persistent swelling that develops into large cankers, gradually killing the bark. These cankers can become infection sites for other pathogens, such as wood-decay fungi. Witches’ brooms sap host resources from other parts of the tree and can ultimately lead to tree death. Also, large brooms are heavy and frequently break under heavy snow, wind, or after branch death, leaving infected trees susceptible to other diseases and secondary pest attacks.

Infection impacts from eastern dwarf mistletoe can differ among its hosts. White and black spruce infection-induced injury and mortality is significant, whereas red spruce can tolerate infection better. One effect of parasitism is a reduction in needle size distal to infections. The magnitude of this reduction is greater in white spruce than red spruce. An infection by eastern dwarf mistletoe on white spruce can reduce the annual radial growth by 79% to 94%.

Eastern dwarf mistletoe can provide extra fuel for forest fires. Large brooms often occur in the lower portion of the crown and are filled with small twigs and dead needles that provide staged fuels and promote the upward spread of fire into tree crowns. Crown fires are far more damaging than surface fires. Brooms broken during winter storms accumulate around the base of infected trees and increase available fuel.

Prevention and management

Pest management strategies for a particular pest vary depending on several factors. These include:

• the population level of the pest (i.e., how numerous the pest is on the affected host[s]);
• the expected damage or other negative consequences of the pest’s activity (either to the host, property, or the environment);
• an understanding of the pest’s life cycle, its various life stages, and the various natural or abiotic agents that affect population levels;
• how many individual host specimens are affected (an individual tree, small groups of trees, plantations, forests);
• the value of the host(s) versus the costs of pest management approaches; and
• consideration of the various silvicultural, mechanical, chemical, biological, and natural control approaches available and their various advantages and disadvantages.

Decisions about pest management strategies require information about each of these factors for informed decision-making. These various factors should then be weighed carefully in terms of costs and benefits before action is taken.

Most infections of eastern dwarf mistletoe on individual or a small number of spruce trees can be removed mechanically by pruning out infected branches or cutting heavily infected trees. This approach works well for small parcels of private land and smaller woodlots where regular monitoring can be completed, and new infections can be quickly removed to reduce the chance of reinfection.

In spruce forests, eastern dwarf mistletoe can only become established if there is an available supply of seeds. Therefore, removing residual infected spruce trees from harvested areas will ensure that subsequent spruce regeneration remains uninfected by the parasitic plant.

Planting a 20-metre buffer of resistant trees, such as white birch or pine species, along the edges of harvested areas adjacent to uncut spruce forests containing eastern dwarf mistletoe, will minimize the risk of re-establishment in the regenerating spruce forest. It is important to monitor these buffers every 3 to 5 years to identify any naturally regenerated host trees that have become infected. Infected regeneration should be cut. This is critical to ensure the parasite does not move back into regenerating areas. Similar approaches apply to areas burnt by wildland fires. When fires skip or only partially burn long-established infection centres, all large or small residual spruce trees should be removed to ensure subsequent spruce regeneration remains parasite-free.

There are no chemical pesticides that have proven effective against eastern dwarf mistletoe without negatively affecting the host tree.

Photos

Dwarf mistletoe plants bearing immature seeds on a spruce twig

Stems of dwarf mistletoe

Witches' broom

Witches'-broom on black spruce formed as a result of infection by Arceuthobium pusillum, the causal agent of eastern dwarf mistletoe.

A witches' broom on white spruce (Picea glauca) caused by an eastern dwarf mistletoe (Arceuthobium pusillum) infection. Felix Cove, NL.

Andrea C. Skinner

A witches' broom formed on a white spruce (Picea glauca) caused by an eastern dwarf mistletoe (Arceuthobium pusillum) infection. You can see the excessive branching near the crown of the host tree. Felix Cove, NL.

Andrea C. Skinner

Close up of a witches' broom on a white spruce (Picea glauca). The broom is caused by an eastern dwarf mistletoe (Arceuthobium pusillum) infection which causes excessive branching on the host. Felix Cove, NL.

Andrea C. Skinner

The underside of a witches' broom. The broom is caused by an infection of eastern dwarf mistletoe (Arceuthobium pusillum) on white spruce (Picea glauca). Felix Cove, NL.

Andrea C. Skinner

Eastern dwarf mistletoe (Arceuthobium pusillum) infection on a black spruce (Picea mariana).

Deceased spruce (Picea) tree with a witches' brook visible among the branches.

Andrea C. Skinner

Eastern dwarf mistletoe (Arceuthobium pusillum) growing from a balsam fir (Abies balsamea) twig.

Selected references

Baker FA. Biology and control of eastern dwarf mistletoe [thesis/dissertation]. University of Minnesota; 1981.

Bonga JM. An unusual witches’ broom on black spruce caused by eastern dwarf mistletoe. For Sci. 1964;10(1):77–78. https://doi.org/10.1093/forestscience/10.1.77

Clark J, Bonga JM. Photosynthesis and respiration in black spruce (Picea mariana) parasitized by eastern dwarf mistletoe (Arceuthobium pusillum). Can J Bot. 1970;48(11):2029–2031. https://doi.org/10.1139/b70-296

Laut JG. Eastern dwarf mistletoe on jack pine in Manitoba. Plant Dis Rep. 1967;51(11):899–900. https://hdl.handle.net/2027/chi.20173039

Hawksworth FG, Wiens D. Dwarf mistletoes: biology, pathology, and systematics. Agricultural Handbook 709. U.S. Dept. of Agriculture, Forest Service. 1996. https://research.fs.usda.gov/download/treesearch/4699.pdf

Mann H. Uncommon wildflowers of Newfoundland 4: dwarf mistletoe (Arceuthobium pusillum Peck). Sarracenia. 2009;17(3):40–42. https://dai.mun.ca/pdfs/sarracenia/SarraceniaVolume173Fall2009.pdf

Singh P. Eastern dwarf mistletoe: distribution and severity in black spruce stands of Newfoundland. Plant Dis. 1982;66(4):312–316. https://www.apsnet.org/publications/plantdisease/backissues/Documents/1982Articles/PlantDisease66n04_312.pdf

Singh P, Carew GC. Impact of eastern dwarf mistletoe in black spruce forests of Newfoundland. Eur J Plant Pathol. 1989;19(5-6):305–322. https://doi.org/10.1111/j.1439-0329.1989.tb00266.x

Reblin JS, Logan BA, Tissue DT. Impact of eastern dwarf mistletoe (Arceuthobium pusillum) infection on the needles of red spruce (Picea rubens) and white spruce (Picea glauca). Tree Physiol. 2006;26(10):1325–1332. https://doi.org/10.1093/treephys/26.10.1325

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