Dwarf mistletoes are parasitic plants that infect conifers. This page covers the basic biology, taxonomy, and impacts of dwarf mistletoes. At the end are a couple of big photo sliders, tables of species, and references. A companion page treats the ecology and management of dwarf mistletoes.
Dwarf mistletoes infect only conifers. In North America, in fact, they only infect one family, Pinaceae. In western North America, many of the most important tree species are hosts. Among the 8 Old World species, three infect Juniperus spp. (Cupressaceae) .
The dwarf mistletoes are in the genus Arceuthobium (order Santalales; Nickrent places the genus in family Viscaceae while the Angiosperm Phylogeny Group 2016 retains it in Santalaceae pending further data [1, 7]. They are all “leafless” (there are minute scale leaves). They are “dwarf” in comparison to true mistletoes (Phoradendron and Viscum spp.). The shoots range up to about 20 cm long for the larger species. The shoots are nonwoody, last only 2-7 yr, and then can regenerate. Like Phoradendron and most Viscum spp., dwarf mistletoes are dioecious.
A very important feature is the mode of dispersal. The fruit literally explodes like a cannon, shooting its seed at an initial velocity of 97 km hr-1 (60 mph) 10 meters or so, up to a maximum of 16 m . You don’t want to be taking a close look at it when that happens!
For years we were settled on about 42 species in the genus. After developing an extensive molecular phylogeny, Nickrent proposed reducing the number of species to 26 . He applied this general concept in the chapter on Viscaceae in the updated Flora of North America , resulting in only 7 species recognized in the USA and Canada. Many of the former species are considered subspecies of A. campylopodum. All 8 Old World species survived . Tables of species are toward the end of the page.
Arceuthobium spp. are not as dependent on particular conditions for reproduction, dispersal, and infection like many fungi are. However, forest stand conditions and disturbance regime can dramatically affect their spread rate, intensification, and long-term prospects. See the companion page for details.
Dwarf mistletoes occur in much of the northern hemisphere, but the greatest diversity is in western North America. Here they occur from boreal Canada and Alaska, south to Guatemala, Belize, and Honduras. Only one species, A. pusillum, occurs in eastern North America, from eastern Saskatchewan to Newfoundland, and south to northern New Jersey and adjacent Pennsylvania. One species, A. bicarinatum, is endemic to the Caribbean island Hispaniola.
Eight species occur outside the Americas [6, 8]. See a table of species near the bottom of the page for their distribution.
Let’s start with a seed flying through the air. Depending on where it lands, a lot of things can happen. If you’re the baby embryo, landing on the ground is bad – your journey on this earth is already finished. Or you land on a non-host, and everything goes well until you penetrate and find you don’t have the keys to the house. Again, game over.
Where you really want to land is on a compatible host, on an upward-pointing needle on a branch segment that is up to about 5 years old (although A. americanum can penetrate the thin bark of Pinus contorta on much older branches). The sticky viscin coat on the seed holds on. But when there is rain or fog, the viscin swells and turns from adhesive to lubricant, and you slide down the needle to the bark surface. Then you hold on tight. If you land on a downward-pointing needle, you fall and get another chance to land on a branch . . . or the ground.
Some tropical species (including southwestern dwarf mistletoe, which reaches up into the USA) will germinate right away. Most temperate species wait for spring. You send out your radicle. When it encounters an obstruction, such as a needle base, it develops a “holdfast” that grips tight. In the middle it sends down a penetration wedge that penetrates the bark by mechanical pressure. This is when you’re glad you’re on a young branch, otherwise you might run out of energy before you get in.
Now comes a period of setting up house inside the host. Establishing an endophytic system (see Anatomy below) provides food and water from the host. Eventually a swelling usually will develop, then 2-5 years after infection, the first shoots appear. After another year or two, flowers appear. Pollen is dispersed by wind or insects to another plant.
The species vary quite a bit in phenology (what time of year they do certain things). Some flower early in the growing season and some rather late. The fruit takes 13-17 months to mature, so seed dispersal occurs the following year, later than flowering. The complete cycle takes at least 6-8 years.
Signs and Symptoms
Being a plant with macroscopic shoots, one would think dwarf mistletoes are easier to detect than are most other diseases. That is not always the case. First consider shoots. It takes at least two years after infection before shoots appear (during that period they are “latent” infections). Second, especially during a drought, some species may drop shoots and they can be very scarce, although the plants are very much alive inside the host. Finally, they are “dwarf”, but some are very small, even shorter than the needles of their host, and very difficult to see except in branches near the ground:
- Arceuthobium pusillum, eastern dwarf mistletoe, shoots average 1 cm long
- A. douglasii, Douglas-fir dwarf mistletoe, averages 2 cm long
Luckily there are other signs and symptoms (see slide carousels below for illustrations):
- When shoots drop, they leave behind basal cups where they were borne on the bark. These are inconspicuous and can only be detected when close.
- Swellings occur at the site of initial infection in most cases. The swellings are fusiform, tapering down at each end. Infections can result in persistent swelling that becomes very large and gradually kills bark (a canker). These cankers can be infection courts for wood-decay fungi.
- The most striking symptom is a witches’ broom, an area of abnormally frequent branching resulting in a cluster of branches. These range from small to large and from loose to tight and dense.
- Resinosis (resin exudation) often occurs around infections
- Dead tops are common in some cases when numerous large brooms in the lower crown divert food away from the top.
- Heavy infection can lead to thin foliage and chlorosis.
- Seed production and seed viability are reduced.
- As intensification of the disease results in more branches infected, growth is reduced and premature mortality may result.
After penetration under the holdfast (see Life Cycle above), the penetration wedge grows down as far as the cambium . Then the endophytic system begins to form (endophytic means inside the plant). First, tissue from the penetration wedge continues to grow radially down to just beyond the cambium. It does not actively penetrate the xylem; it waits for the xylem to grow out around it, year by year. This structure becomes the first sinker. One can age mistletoe infections precisely by counting annual growth rings from the deepest sinker outward.
But wait – if the cambium adds a new layer of wood, and the sinker is inside the wood and the rest of the mistletoe outside, why doesn’t this stretch and tear the sinker away from the rest? The mistletoe has a very sophisticated mechanism to deal with this. Where the sinker penetrates the host cambium, it develops a little stretching and growing zone. This can be called an intercalary meristem. Intercalary means it is inside, along the length. It’s growth is synchronized precisely with that of the cambium.
Another kind of organ grows out parallel to the cambium. These are cortical strands. These are mostly longitudinal but also circumferential. They ramify through the phloem, where they begin to appropriate organic nutrients.
The cortical strands send down additional sinkers as they grow. The cortical strands plus the sinkers comprise the endophytic system. Now the mistletoe is set for life.
Localized vs. systemic infections
Localized infections remain in a limited section of the host branch . A fusiform swelling usually forms there, extending about as far as the endophytic system. All dwarf mistletoes can form localized infections.
Some dwarf mistletoes can become systemic (within a branch system). The endophytic system grows acropetally (toward the branch tip) faster than basipetally. Swelling is generally restricted to the area of initial infection. Eventually they reach just behind the apical meristem at the branch tip. In a more intimate form of synchronization, they keep up with tip growth and and develop into new branches as they are produced. This systemic infection is similar to Elytroderma needle cast of ponderosa pine. The mistletoe can then produce shoots anywhere along the systemically infected branch system.
Systemic infections are considered to represent a more advanced evolutionary state, and are generally associated with small shoots. In North America, they are consistently formed only by A. americanum, A. douglasii, A. pusillum, and A. guatemalense. Brooms are generally formed in both kinds of infection.
The dwarf mistletoes have no significant leaves, but have some chlorophyll in their stems and can photosynthesize there. And why shouldn’t they? These parasites evolved from photosynthetic, free-living ancestors, so the ability is already there. Those stems are up there in the canopy, so why not take advantage of that extra energy, given that the photosynthetic machinery is operative.
However, they have less chlorophyll than true mistletoes, corresponding with an often sickly green or yellow color. They take more photosynthate from the host than do true mistletoes. They are considered on the brink of being holoparasites . And indeed, in old stem infections it is not uncommon for the pathogen to survive for decades or more while producing no shoots, which is certainly holoparasitism. This is one reason they are more damaging than true mistletoes when there are many infections on a tree.
In western North America, dwarf mistletoes are generally considered the most damaging type of forest disease. There are some areas where root diseases are probably more important, but mistletoes are important in many forest types, on most major tree species, over vast areas. Important forms of damage include:
- growth loss
- predisposition to other mortality agents (e.g., bark beetles) and decay fungi (after old stem infections become cankerous)
- dead tops
- defect (stem swellings, cankers)
- Reduced production and viability of seed
An animation of the modeled impacts of Douglas-fir dwarf mistletoe shows the results of disease progression . With moderate amounts of dwarf mistletoe at age 80, after 100 years the stand actually has lower basal area than it started with! Young trees are particularly vulnerable to permanent deformity and mortaliity. A seedling or young sapling that is infected early often has a crown that is all witches’ broom, and will remain stunted for life. But even mature trees have reduced lifespan and growth potential when severely infected. Host-pathogen combinations vary in lethality.
Significant growth impact is seldom seen until at least the lower 1/3 to 1/2 of the crown is heavily infected. Light infection throughout the crown or heavy infection in just the lower crown usually cause little or no damage, except perhaps when large brooms are produced or stem infections cause problems. Brooms seem to have a special effect on growth. A large broom or several smaller ones can devastate the rest of the crown, leading to severe crown thinning.
It has been estimated that, in western USA, dwarf mistletoes cause direct, annual wood volume loss of 11.8 million m3 . This is roughly equivalent to 3.3 billion board feet. This estimate includes only commercial forest land on National Forests, and it doesn’t include defect and loss of wood quality. There is a great deal more dwarf mistletoe in non-timber lands and in other ownerships. That said, most losses are not realized, i.e., based on current management, much of it likely wouldn’t have been harvested even if healthy. In western Canada, a comparable loss estimate is 3.8 million m3 yr-1 .
In the US study  the highest losses on an area basis were about 1.5 m3 ha-1 yr-1 for Pseudotsuga menziesii and Larix occidentalis in certain regions.
To put this in perspective, 11.8 million m3 is:
- enough wood to build 1.1 million small, 3-bedroom houses each year (3000 bd ft each)!!!
- over 13% of the 2016 harvest volume from the same western states (87.9 million m3) . This is an underestimate because the harvest data include non-USFS land.
- From an ecological perspective, it is enough wood to make trees a lot larger and older if they were neither infected nor harvested.
- Dwarf mistletoes infect only conifers, in fact, only one family (Pinaceae) in North America.
- They are all “leafless”, that is, the leaves are minute scales.
- They have chlorophyll, but depend on host for most carbohydrates (as well as all water and inorganic nutrients).
- They are “dwarf” in comparison to true mistletoes (Phoradendron and Viscum spp.). The shoots range up to about 20 cm long for the larger species.
- The part inside the host plant is the endophytic system, consisting of
- cortical strands, ramifying through the phloem
- sinkers following rays into the xylem
- Shoots are nonwoody, last a few years, and then can regenerate.
- Like Phoradendron and most Viscum spp., they are dioecious (separate male and female plants).
- A very important feature is the mode of dispersal: the berries explode, shooting sticky seeds at an initial velocity of 97 km hr-1 (60 mph) 10 meters or more.
- Tend to be host specific, which is important in management.
- They are among the most damaging forest pathogens in many parts of western North America.
Nickrent [7, 8] used for taxonomy; other data mostly from Hawksworth & Wiens  and Ciesla et al. .
Dwarf mistletoes of USA and CanadaChoose to page through or show as many rows as you like.
|Species||Subspecies||Common name||Principal hosts||Distribution|
|A. americanum||lodgepole pine d. m.||Pinus contorta and P. banksiana||British Columbia to Manitoba and south to California and Colorado|
|A. divaricatum||piñon d. m.||P. monophylla and P. edulis||southwestern USA and Baja California|
|A. campylopodum||campylopodum||western d. m.||P. ponderosa||Washington and Idaho south to Baja California|
|abietinum||fir d. m.||Abies concolor and A. magnificae (with host-specialized formae specialis)||Washington and Idaho south to California and Arizona, Chihauhua|
|apachecum||Apache d. m.||P. strobiformis||Arizona, New Mexico, Chihuahua|
|blumeri||Blumer's d. m.||P. strobiformis||Arizona, Mexico|
|californicum||sugar pine d. m.||P. lambertiana||California|
|cyanocarpum||limber pine d. m.||P. flexilis, P. aristata, P. albicaulis, and P. longaeva||Oregon, Idaho, Montana south to California, Nevada, Utah, Colorado|
|laricis||larch d. m.||Larix occidentalis and Tsuga mertensiana||British Columbia, Washington, Oregon, Idaho, Montana|
|littorum||coastal d. m.||P. radiata and P. muricata||coastal California|
|microcarpum||western spruce d. m.||Picea engelmannii, P. pungens, Pinus aristata||Arizona and New Mexico|
|monticola||western white pine d. m.||Pinus monticola||small area around western border between Oregon and California|
|occidentale||digger pine d. m.||P. sabiniana||California|
|siskiyouense||knobcone pine d. m.||P. attenuata||small area around western border between Oregon and California|
|tsugense||hemlock d. m. (includes both subspp. of the former A. tsugense)||Abies amabalis, A. lasiocarpa, A. procera, Tsuga heterophylla, T. mertensiana||Alaska and British Columbia south to California and Idaho|
|A. douglasii||Douglas-fir d. m.||Pseudotsuga douglasii||California east to Colorado, New Mexico, west Texas, and south to Baja California|
|A. pusillum||eastern d. m.||Picea mariana, P. glauca, and P. rubens||eastern Saskatchewan to Newfoundland, and south to northern New Jersey and adjacent Pennsylvania|
|A. gillii||Chihuahua pine d. m.||Pinus leiophylla var. chihuahuana, P. lumholtzii, P. herrerai||southeastern Arizona and southwestern New Mexico south into Mexico|
|A. vaginatum||cryptopodum (subsp. vaginatum is restricted to Mexico)||southwestern d. m.||Pinus ponderosa var. scopulorum, P. arizonica, P. engelmannii||Utah and Colorado south to northern Mexico|
Dwarf mistletoes outside the Americas
|Species||Common name||Principal hosts||Distribution|
|A. azorica||Azores d. m.||Juniperus brevifolia||Azores|
|A. chinense||Keteleeria d. m.||Keteleeria evelyniana (Pinaceae)||Yunnan and Sichuan provinces of southern China|
|A. juniperi-procerae||East African d. m.||J. procera||East African nations of Kenya, Ethiopia, and Eritrea|
|A. minutissimum||Himalayan d. m.||Pinus wallichiana||southern Himalayas (Pakistan, India, Nepal, Bhutan)|
|A. oxycedri||juniper d. m.||Juniperus spp., esp. J. oxycedrus, and a variety of introduced species||the largest range of the genus, extending from Spain and Morocco to the Himalayas of Tibet|
|A. pini||alpine d. m.||Pinus spp.||southwestern China|
|A. sichuanense||Sichuan d. m.||Picea spp.||southwestern China, Tibet, and Bhutan|
|A. tibetense||Tibetan d. m.||Abies spp.||small part of eastern Tibet|
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- 2.Calvin CL, Wilson CA. 1996. Endophytic system. In: Dwarf Mistletoes: Biology, Pathology and Systematics. Agricultural Handbook 709, eds Hawksworth FG, Wiens D, pp. 113–122. Washington, DC: USDA Forest Service.
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- 4.Drummond DB. 1982. Timber loss estimates for the coniferous forest of the United States due to dwarf mistletoes. Report 83–2. USDA Forest Service, Forest Pest Management, Methods Application Group, Fort Collins, Colorado, USA <https://books.google.com/books?id=bZjlc-6icacC>.
- 5.Hawksworth FG, Hinds TE. 1964. Effects of dwarf mistletoe on immature lodgepole pine stands in Colorado. Journal of Forestry 62:27–32 <10.1093/jof/62.1.27>.
- 6.Hawksworth FG, Wiens D, Geils BW, Nisley RG. 1996. Dwarf Mistletoes: Biology, Pathology and Systematics. Agricultural Handbook 709. Washington, DC: USDA Forest Service. 410 pp.
- 7.Nickrent DL. 2016. Viscaceae Batsch: The Christmas Mistletoe Family. In: Flora of North America, ed Flora of North America Editorial Committee 12:422–440. New York, New York: Oxford University Press <https://nickrentlab.siu.edu/NickrentPDFs/Nickrent2016FNAs.pdf>.
- 8.Nickrent DL, García MA, Martín MP, Mathiasen RL. 2004. A phylogeny of all species of Arceuthobium (Viscaceae) using nuclear and chloroplast DNA sequences. American Journal of Botany 91(1):125–138 <10.3732/ajb.91.1.125>.
- 9.Oswalt SN, Smith WB, Miles PD, Pugh SA. 2019. Forest Resources of the United States, 2017: A Technical Document Supporting the Forest Service 2020 RPA Assessment. General Technical Report WO-GTR-97. USDA Forest Service, Washington, DC <https://www.fs.usda.gov/treesearch/pubs/57903>.
- 10.Worrall JJ, Geils BG. 2008. Dwarf mistletoes. The Plant Health Instructor, American Phytopathological Society <10.1094/PHI-I-2006-1117-01>.