Lodgepole pine dwarf mistletoe

Written by J.P. Brandt Revision 2024

French disease name: Faux-gui du pin tordu latifolié
Pathogen name: Arceuthobium americanum Nutt. Ex Engelm.
Kingdom: Plantae
Phylum: Tracheophyta
Class: Santalales
Order: Santalales
Family: Viscaceae

General information and importance

Lodgepole pine dwarf mistletoe is a highly evolved obligate parasitic plant. It has greatly reduced leaves and no apparent root system. The plant has a specialized vascular system that allows it to rob its tree host of water, carbohydrates, and nutrients through cellular connections at the point of infection.

Lodgepole pine dwarf mistletoe is second only to mountain pine beetle (Dendroctonus ponderosae) in the volume of wood lost in western Canada to its two principal hosts, lodgepole pine (Pinus contorta var. latifolia) and jack pine (P. banksiana). Older, severely infested stands have dead or weakened trees with poor growth and stem form. Tree density can also be quite low in forest stands from mortality caused by this parasitic plant.

Forest management practices must be adjusted where this pest occurs to minimize the potential infection of regenerating stands following harvest.

Distribution and hosts

Lodgepole pine dwarf mistletoe is the most widely distributed species of dwarf mistletoe in North America. In Canada, it is currently found from eastern Manitoba to British Columbia. An isolated infestation of jack pine found in northwestern Ontario, near Lac Seul, in the 1960s has been extirpated. Since then, additional lodgepole pine dwarf mistletoe infestations have not been found in Ontario.

In Canada, the primary hosts of A. americanum include:

jack pine in Alberta, Saskatchewan, and Manitoba
lodgepole pine in Alberta and British Columbia
shore pine (Pinus contorta var. contorta) in coastal British Columbia

Occasional hosts include ponderosa pine (P. ponderosa) in British Columbia, and whitebark pine (P. albicaulis) and limber pine (P. flexilis), in the United States. Rare hosts include white spruce (Picea glauca) and black spruce (P. mariana).

The parasitic plant’s northern distribution is limited by extremely cold winter temperature (about -40°C), which kills overwintering seeds.

Host parts affected

Twigs, branches, and occasionally stems.

Symptoms and signs

The most evident symptom of attack by lodgepole pine dwarf mistletoe is abnormal, dense branching known as witches’ brooms. Broom shape and size vary with factors such as host, age, and point of infection. Brooms on lodgepole pine tend to be denser than brooms on jack pine.

There are two types of infections:

localized infections, resulting in a spindle-shaped swelling of the twig or branch; and
systemic infections, causing brooms.

Green to yellow-green aerial shoots up to 10 centimetres in length and about 1 to 1.5 millimetres in diameter can be found at localized infections. Aerial shoots along the branches of brooms tend to be shorter and focussed at branch nodes.

Lodgepole pine dwarf mistletoe plants are either male or female. Male flowers are small (about 2 millimetres across), three petaled, and bright yellow. They occur for several weeks very early in the spring from late March until the end of April (when some snow can still be found on the ground). Female plants flower at the same time, but the flowers are inconspicuous. A single tree can harbour hundreds of individual dwarf mistletoe infections.

By late July and early August, mature oval berries can be found on female plants. Each green berry contains a single seed, which discharges explosively when the berry is squeezed. Shoots can remain on the host for several years; however, they sometimes die or are cast in the fall. Basal cups, which are present where the shoot emerges from the host, also remain and can be found at the surface of the twig or branch. Aerial shoots of dwarf mistletoe are generally not visible for 2 to 3 years after initial infection, so surveys for dwarf mistletoe must consider these latent infections.

There are several other species of dwarf mistletoe in Canada (larch dwarf mistletoe, A. laricis; Douglas-fir dwarf mistletoe, A. douglasii; eastern dwarf mistletoe, A. pusillum; and western hemlock dwarf mistletoe, A. tsugense). These species, however, occur on principal conifer hosts other than lodgepole or jack pine.

Witches’ brooms can also be a symptom of Elytroderma needle disease, caused by the fungus Elytroderma deformans on lodgepole pine, jack pine, and ponderosa pine. It can be differentiated by the absence of the small shoots of dwarf mistletoe plants on swollen twigs and branches. It can also be identified when the fungus is present on infected needles.

Disease cycle

Lodgepole pine dwarf mistletoe has a similar disease cycle on lodgepole pine and jack pine, but differs in overall duration; about 7 years on lodgepole pine and about 5 years on jack pine.

Seeds discharge from female plants in August and September with substantial force because of hydrostatic pressure build-up in maturing berries. Berries fall when they are ripe and enable seed ejection and dispersal up to 10 to 12 metres. Needle-bearing twigs intercept most seeds by chance because these provide the largest targets for dispersing seeds. Seeds are covered with a viscous layer that acts both as an adhesive and a lubricant. When the viscous layer is dry, it holds the seed to anything it lands on. When it is wet, it allows the seeds to slide down the needles. Seeds landing on needles located along the upper sides of twigs and branches typically slide down to the base of needles where they overwinter. Seeds that are intercepted by needles along the lower sides typically slide off either to another part of the host or to the ground. Seeds germinate the following spring in May (range April to June). About 1 month later, a holdfast (specialized structure of the germinating seed that attaches to and penetrates the host) develops at the radicle tip where it has been obstructed. Host penetration begins by August.

About 2 years later, a swelling of the twig at the point of penetration (the first symptom of infection) becomes visible. Aerial shoots emerge about 1 month later, usually near the holdfast (if still attached). The parasite’s endophytic system (the system of vascular tissues of the parasite within the host) is well established at this point.

During the third growing season, shoots continue to elongate. By the end of the season, shoots of both sexes are mature. In the subsequent growing season, male and female plants flower during late March or April until May. Mature fruits develop on female plants by the end of the growing season to repeat the disease cycle.

The fungus Caliciopsis arceuthobii (syn.: Wallrothiella arceuthobii) commonly infects the flowers of female plants and ultimately affects dwarf mistletoe seed production. This provides a natural biological control of the plant. Presence of this fungus is evident as shiny black perithecia (fruiting bodies of the fungus) that appear on female flowers in the spring. In addition, resin disease is common on A. americanum infecting lodgepole pine in the Rocky Mountains. The symptoms of resin disease include:

excessive resinosis at the dwarf mistletoe swelling,
necrotic lesions and discoloration of the host bark,
necrophylactic periderm, and
dead dwarf mistletoe shoots.

Resin disease appears to be a disease complex caused by the fungi Alternaria alternata, Aurobasidium pullulans, and Epicoccum nigrum.

Damage

Heavy infections caused by lodgepole pine dwarf mistletoe substantially reduce wood quality, tree diameter and height growth, and in many cases, result in the death of the host tree. Dead twigs, branches, and treetops from dwarf mistletoe infection also provide entry points for decay fungi and other secondary pests, further negatively affecting the host. Infected branches frequently break due to decay, excessive broom size, or snow load on large brooms. The latter situation can present a hazard in high-use recreational sites.

Stands where dwarf mistletoe has been established for a long time usually contain infection centres where tree density is low because of mortality and where most young understory trees are heavily infected. In western Canada where this parasitic plant is quite common, it is second only to mountain pine beetle in the amount of damage it causes to its two primary pine hosts.

The presence of dwarf mistletoe brooms can result in intensification of wildfire because the witches’ brooms act as ladder fuel, which allows fire to transition from the ground to the canopy.

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 against a particular pest.

For lodgepole pine dwarf mistletoe, most infections on individual or a small number of pine trees can be removed mechanically by pruning 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 detected and removed to reduce the chance of reinfection. This approach is not practical at the forest stand scale.

In lodgepole and jack pine forests, lodgepole pine dwarf mistletoe can only become established if seeds are available. Therefore, removing residual overstory infected pine trees from harvested areas will ensure that subsequent pine regeneration remains free of the parasitic plant.

Along the edges of harvested areas where lodgepole pine dwarf mistletoe is found in adjacent uncut pine forests, planting a 20-metre-wide buffer of white or black spruce (which are rarely infected) will minimize the risk of re-establishment of the parasitic plant in the regenerating pine forest. It is important to monitor these buffers every 3 to 5 years to identify naturally regenerated lodgepole pine or jack pine that have become infected. Infected regeneration should be cut to ensure the parasite does not move back into regenerating pine areas. Positioning of cut block boundaries to incorporate barriers to seed dispersal, such as roads, is also a means of preventing establishment in regenerating stands. Similar approaches apply to areas burned by wildland fires. When fires skip or only partially burn long-established dwarf mistletoe infection centres, all large or small infected residual pine trees should be removed to ensure subsequent pine regeneration remains parasite free.

Several chemical herbicides and growth regulators have been evaluated against dwarf mistletoes; however, all but one of these were ineffective against parasitic plants without also negatively affecting the host tree. The most promising chemical was Ethephon [(2-chloroethyl) phosphonic acid], which caused early abscission of flowers, fruits and shoots but did not kill the endophytic system. The latter active ingredient has been registered and used in the United States against dwarf mistletoes on ornamentals. In Canada, pesticides registered for use against lodgepole pine dwarf mistletoe under specific situations may change from year to year. Therefore, please search Health Canada’s Pesticide Product Information Database for currently registered pesticides and product information for use against this parasitic plant. The application of any registered product should be based on population size and applied only when necessary and against the approved life stage. It is also recommended to consult a local tree care professional. Pesticides may be toxic to humans, animals, birds, fish, and other beneficial insects. Apply registered products only as necessary and follow all directions and precautions noted on the manufacturer’s label. In some jurisdictions and situations, only a licensed professional can apply pesticides. Consulting relevant local authorities to determine local regulations that are in place is recommended.

Selected references

Baranyay, J.A.; Safranyik, L. 1970. Effect of dwarf mistletoe on growth and mortality of lodgepole pine in Alberta. Canada Department of Fisheries and Forestry, Canadian Forestry Service. Ottawa, Ontario. Publication 1285. 24 p.

Baranyay, J.A.; Smith, R.B. 1972. Dwarf mistletoes in British Columbia and recommendations for their control. Environment Canada, Canadian Forestry Service, Pacific Forest Research Centre.Victoria, British Columbia. Information Report BC-X-72. 18 p.

Brandt, J.P.; Brett, R.D.; Knowles, K.R.; Sproule, A. 1998. Distribution of severe dwarf mistletoe damage in west-central Canada. Natural Resources Canada, Canadian Forest Service. Northern Forestry Centre. Edmonton, Alberta. Special Report 13. 27 p.

Brandt, J.P.; Hiratsuka, Y.; Pluth, D.J. 2004. Extreme cold temperatures and survival of overwintering and germinated Arceuthobium americanum seeds. Canadian Journal of Forest Research 34(1): 174–183. https://doi.org/10.1139/x03-200

Brandt, J.P.; Hiratsuka, Y.; Pluth, D.J. 2005. Germination, penetration, and infection by Arceuthobium americanum on Pinus banksiana. Canadian Journal of Forest Research 35(8): 1914–1930. https://doi.org/10.1139/x05-113

Brandt, J.P. 2006. Life cycle of Arceuthobium americanum on Pinus banksiana based on inoculations in Edmonton, Alberta. Canadian Journal of Forest Research 36(4): 174–183. https://doi.org/10.1139/x05-288

British Columbia Ministry of Forests, Lands, and Natural Resource Operations. 1995. Dwarf mistletoe management guidebook. http://www.quickscribe.bc.ca/secure/repository/696_!Dwarf-Mistletoe.html [Accessed April 2024]

Frankel, S.; Adams, D. 1989. Reduction of dwarf mistletoe with the plant growth regulator ethephon. United States Department of Agriculture, Forest Service, Pacific Southwest Region, Forest Pest Management. Report No. 89-1.

Hawksworth, F.G. 1965. Life tables for two species of dwarfmistletoe. I. Seed dispersal, interception, and movement. Forest Science 11(2): 142–151. https://doi.org/10.1093/forestscience/11.2.142

Hawksworth, F.G.; Wiens, D. 1996. Dwarf mistletoes: biology, pathology, and systematics. United States Department of Agriculture, Forest Service. Washington, D.C. Agricultural Handbook 709. 410 p.

Jerome, C.A.; Ford, B.A. 2002. The discovery of three genetic races of the dwarf mistletoe Arceuthobium americanum (Viscaceae) provides insight into the evolution of parasitic angiosperms. Molecular Ecology 11(3): 387–405. https://doi.org/10.1046/j.0962-1083.2002.01463.x

Mark, W.R.; Hawksworth, F.G.; Oshima, N. 1976. Resin disease: a new disease of lodgepole pine dwarf mistletoe. Canadian Journal of Forest Research 6(3): 415–424. https://doi.org/10.1139/x76-055

Mathiasen, R.L. 2021. Mistletoes of the Continental United States and Canada. Brit Press, Fort Worth Botanic Garden. Fort Worth, Texas. 220 p.

Ramsfield, T.D.; Shamoun, S.F.; van der Kamp, B.J. 2009. The phenology and impact of Caliciopsis arceuthobii on lodgepole pine dwarf mistletoe, Arceuthobium americanum. Botany 87(1): 43–48. https://doi.org/10.1139/B08-089

Smith, R.B. 1974. Infection and development of dwarf mistletoes on plantation-grown trees in British Columbia. Environment Canada, Canadian Forestry Service, Pacific Forest Research Centre. Victoria, British Columbia. Information Report BC-X-97. 21 p.

Cite this fact sheet

Brandt, J.P. 2024. Lodgepole pine dwarf mistletoe. In J.P. Brandt, B. Daigle, J.-L. St-Germain, A.C. Skinner, B.C. Callan, and V.G. Nealis. (editors). Trees, insects, mites, and diseases of Canada’s forests. Natural Resources Canada, Canadian Forest Service, Headquarters. Ottawa, Ontario.

Photos

Young lodgepole pine with multiple brooms caused by lodgepole pine dwarf mistletoe.

Bob Erickson

Stem swelling on lodgepole pine caused by lodgepole pine dwarf mistletoe.

Eric Allen

Female lodgepole pine dwarf mistletoe plant with mature berries.