Arceuthobium tsugense (hemlock dwarf mistletoe)

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Habit

A. tsugense on host, Horne Lake, British Columbia, Canada. Note large fusiform swelling on branch of Pinus contorta subsp. contorta.

Daniel L. Nickrent

Host trees

Infection of three host tree species: Abies lasiocarpa, Tsuga mertensiana and Pinus albicaulis. Lane County, Oregon, USA.

Daniel L. Nickrent

Identity

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Preferred Scientific Name

Arceuthobium tsugense (Rosendahl) G.N.Jones

Preferred Common Name

hemlock dwarf mistletoe

Other Scientific Names

Arceuthobium douglasii var. tsugense (Rosendahl) M.E.Jones
Razoumofskya tsugensis Rosendahl

International Common Names

English: dwarf mistletoe: hemlock; mountain hemlock dwarf mistletoe; western hemlock dwarf mistletoe

EPPO code

ARETS (Arceuthobium tsugense)

Summary of Invasiveness

Top of page Arceuthobium spp. do not spread rapidly and cannot be considered highly invasive. They do, however, constitute a serious threat as a result of their ability to build up gradually over the life of a forest and cause severe damaging effects on a number of important forest species.

Their potential to establish in other areas is limited by the need for the living parasite to survive on the pathway and reproduce after entry. Nevertheless, the risk of economic impact is considerable if host species are available. The conifers at greatest risk would be species, known to be hosts, planted as exotics in other continents, but there is also a certain possibility of spread to related species, not known to be hosts.

Taxonomic Tree

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Domain: Eukaryota
Kingdom: Plantae
Phylum: Spermatophyta
Subphylum: Angiospermae
Class: Dicotyledonae
Order: Santalales
Family: Viscaceae
Genus: Arceuthobium
Species: Arceuthobium tsugense

Notes on Taxonomy and Nomenclature

Top of page A detailed discussion of the taxonomy and taxonomic history of the genus Arceuthobium is provided by Hawksworth and Wiens (1996). The genus Arceuthobium is a member of the plant family Viscaceae and is a clearly defined group of small (generally less than 20 cm high), variously coloured flowering plants that are aerial parasites on conifers of the families Pinaceae and Cupressaceae. They are considered to be the most evolutionarily specialized genus of the family Viscaceae. Arceuthobium has been previously included in the subfamily Viscoideae of the family Loranthaceae, but the subfamilies Loranthoideae and Viscoideae are now generally considered to have family status (Loranthaceae and Viscaceae).

A. tsugense is in subgenus Vaginata, section Campylopoda, series Campylopoda.

Subsp. mertensianae (mountain hemlock dwarf mistletoe) is distinguished on Tsuga mertensiana, in the western USA, while the nominate subspecies (western hemlock dwarf mistletoe) occurs in the same area on T. heterophylla through British Columbia, Canada up to the southern tip of Alaska, USA. The two subspecies can be differentiated by electrophoretic methods (Nickrent and Stell, 1990), but the so-called shore pine race does not appear distinct.

Description

Top of page Like other Arceuthobium spp., A. tsugense is an obligate parasite with an endophytic 'root' system ramifying within the host branch. This endophyte expands within the cortex and becomes embedded in the xylem for some years before aerial shoots are produced, encircling the infected branch and growing along it. The purplish shoots of A. tsugense are usually 5 to 7cm high, but they can be has tall as 13 cm. Internodes are 6 times as long as wide. A. tsugense subsp. tsugense and subsp. mertensianae are similar morphologically but shoot heights vary from 3-13 (mean 7) mm in subsp. tsugense to a mean of 5 mm in mertensianae. Staminate (male) flowers are 2.8 mm across with three or four perianth segments. Pistillate (female) flowers are about 1 mm across. The mature fruit is 3.0 x 2.0 mm (Hawksworth and Wiens, 1996).

Plant Type

Top of page Parasitic
Perennial
Seed propagated

Distribution Table

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The distribution in this summary table is based on all the information available. When several references are cited, they may give conflicting information on the status. Further details may be available for individual references in the Distribution Table Details section which can be selected by going to Generate Report.

Last updated: 23 Apr 2020

Continent/Country/Region	Distribution	Origin	Invasive	Reference
Europe
Slovenia	Absent			EPPO (2020)
North America
Canada	Present, Localized			EPPO (2020)
-British Columbia	Present	Native	Invasive	Hawksworth and Wiens (1996); EPPO (2020)
United States	Present, Localized			EPPO (2020)
-Alaska	Present	Native	Invasive	Hawksworth and Wiens (1996); EPPO (2020)
-California	Present	Native	Invasive	Hawksworth and Wiens (1996); EPPO (2020)
-Oregon	Present	Native	Invasive	Hawksworth and Wiens (1996); EPPO (2020)
-Washington	Present	Native	Invasive	Hawksworth and Wiens (1996); EPPO (2020)

Risk of Introduction

Top of page The risk presented by Arceuthobium spp. introductions into other areas of the world is related to the availability of their hosts. The most obvious risk arises from the fact that several North American hosts (for example, Pinus contorta, P. ponderosa, Tsuga spp. and Pseudotsuga menziesii) have been more or less widely planted in other continents, in the absence of these mistletoes (curiously, Pinus radiata, one of the North American pines most widely planted around the world, is hardly reported as an Arceuthobium host, nor is Picea sitchensis, much planted in parts of Europe). Conversely, the European or Asian hosts of Arceuthobium have not been substantially planted outside their natural range. A secondary risk is that, although in their natural range Arceuthobium spp. occur rather rarely on species other than their main hosts, there is limited data suggesting that they may readily infect some exotic species. There is accordingly a certain risk that Arceuthobium spp. may spread to and affect such exotic hosts if they are introduced into other continents, e.g. P. sylvestris in Europe, Juniperus virginiana in North America.

The risk of accidental introduction is already well recognized and trade in conifer plants is correspondingly controlled in many countries. Exotic Arceuthobium species are also specifically listed as prohibited imports in the European Union, other European countries, Australia, New Zealand, Turkey, Tanzania and no doubt many others. North American countries similarly restrict import of conifers.

Habitat

Top of page A. tsugense is limited to forests in which its specific hosts (certain Abies and Tsuga spp.) are present in substantial numbers.

Habitat List

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Category	Sub-Category	Habitat	Presence	Status
Terrestrial
	Terrestrial – Managed	Managed forests, plantations and orchards	Present, no further details	Harmful (pest or invasive)
	Terrestrial ‑ Natural / Semi-natural	Natural forests	Present, no further details	Harmful (pest or invasive)

Hosts/Species Affected

Top of page A. tsugense occasionally occurs on conifers other than Tsuga spp., when growing nearby. Abies amabilis, A. lasiocarpa and A. procera are likely to be infected. A. tsugense is found very rarely on A. grandis, Pinus contorta var. latifolia, P. albicaulis, P. monticola, Picea breweriana, Picea engelmannii, Picea sitchensis, Pseudotsuga menziesii. The two subspecies rarely infect each other's typical Tsuga host, in the areas where the two occur together. A population of subsp. tsugense, known as the "shore pine race", occurs on Pinus contorta var. contorta in the vicinity of Vancouver Island; it is hardly distinguishable morphologically, but rarely cross-infects.

Host Plants and Other Plants Affected

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Plant name	Family	Context
Abies amabilis (Pacific silver fir)	Pinaceae	Other
Abies lasiocarpa (rocky mountain fir)	Pinaceae	Other
Abies procera (noble fir)	Pinaceae	Other
Pinus contorta (lodgepole pine)	Pinaceae	Other
Pinus lambertiana (big pine)	Pinaceae	Other
Tsuga heterophylla (western hemlock)	Pinaceae	Main
Tsuga mertensiana (Patton's hemlock)	Pinaceae	Main

Growth Stages

Top of page Vegetative growing stage

List of Symptoms/Signs

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Sign	Life Stages	Type
Stems / distortion
Stems / witches broom

Biology and Ecology

Top of page Genetics

Chromosome number 2n = 28 (Hawksworth and Wiens, 1996).

Physiology and Phenology

Arceuthobium spp. are obligate parasites, depending on an endophytic system within the host branch to draw water and nutrients from their hosts. Germination usually occurs in the spring following autumn dispersal and is favoured by light. The seedling shows negative phototropism, causing it to bore directly into the host shoot, even from below. Seedlings of most Arceuthobium spp. can only penetrate young branches less than 5 years old. Most Arceuthobium spp. have no phloem tissue. Transfer of nutrients, including sugars, may depend on close association of host and parasite parenchyma cells, and apoplastic movement via the walls of these cells. Graniferous tracheary elements could also be involved (see Hawksworth and Wiens (1996), for detailed discussion on this topic).

Photosynthesis is apparently important in supporting the seedling as it germinates and attaches, but for the next 2-7 years (usually 3-4) of its life, the parasite persists only as the endophyte inside the host tissue without any aerial shoot. Even after emergence of the aerial shoots, the established parasite has a relatively low photosynthetic capacity, usually much less than 50% of 'normal'.

Once emerged, the parasite shoots of Arceuthobium spp. produce fruits annually, for at least 2 years, and often for 5 years or more (Hawksworth and Wiens, 1996).

Reproductive Biology

A. tsugense, like other Arceuthobium spp., is dioecious. Pollination appears to be predominantly due to insects (especially ants and flies) but may also occur by wind (Hawksworth and Wiens, 1996). Anthesis occurs between July and September in A. tsugense subsp. tsugense, which is about 1-2 weeks earlier than in subsp. mertensianae. However, the seed dispersal period for subsp. tsugense (late September to early November) is about 2-4 weeks later than for subsp. mertensianae. No true seed is formed, as there is no testa, but the embryo is embedded in chlorophyllous endosperm, surrounded by viscin. This will be referred to as a seed for convenience. The embryo is green, a few millimetres long, and has a meristematic radicular apex without a root cap. Dispersal of the seed is exceptional, involving a hydrostatic, explosive process which expels the seed for some distance from the parent plant. Most dispersal occurs as temperatures rise and humidity declines in the morning. The viscin ensures that it is retained by any host shoot that is hit, but if this is a needle, it may slide down with gravity to the base of the needle and germinate there. Although this is the main means of dispersal over a short range, long-distance dispersal also occurs as a result of seeds sticking to birds or mammals. However, any seeds that are ingested by animals are destroyed. Seeds of Arceuthobium spp. do not generally show dormancy and germination normally occurs in the first season after dispersal, though seeds may retain dormancy for 1-4 years when stored in ideal conditions.

Environmental Requirements

The main environmental constraint on an Arceuthobium sp. is the presence of its host, which is in turn determined by multiple environmental requirements. The different North American species most obviously differ in the latitudinal limits of their range, from those that occur in Canada and northern US states, to those which are confined to Mexico, with all intermediates. Species also differ from those with an essentially coastal distribution to those with a continental distribution. The relevant factors further interact to determine an altitudinal range, reflecting the fact that conifers form a distinctive element of montane vegetation. Soil conditions have practically no importance.

Air Temperature

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Parameter	Lower limit	Upper limit
Mean annual temperature (ºC)	1	15
Mean maximum temperature of hottest month (ºC)	18	32
Mean minimum temperature of coldest month (ºC)	-26	-2

Rainfall

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Parameter	Lower limit	Upper limit	Description
Dry season duration	2	12	number of consecutive months with <40 mm rainfall
Mean annual rainfall	200	1150	mm; lower/upper limits

Rainfall Regime

Top of page Uniform

Natural enemies

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Natural enemy	Type	Life stages	Specificity	References	Biological control in	Biological control on
Caliciopsis arceuthobii	Pathogen
Glomerella cingulata	Pathogen
Neonectria neomacrospora	Pathogen

Notes on Natural Enemies

Top of page Caliciopsis arceuthobi and Colletotrichum gloeosporiodes are major fungal parasites of Arceuthobium spp. in North America that are found on A. tsugense in the USA and Canada (Hawksworth and Wiens, 1996).

Kope et al. (1997) suppose that Glomerella cingulata could be an important control measure.

Means of Movement and Dispersal

Top of page Natural Dispersal (Non-Biotic)

Natural dispersal of Arceuthobium spp. is by the explosive fruits which can expel the seeds at speeds of 2.6 m/s up to a 15-m distance (Hinds and Hawksworth, 1965). In spite of this, the natural spread may not exceed about 1.5 m/annum (Hawksworth, 1958).

Vector Transmission (Biotic)

Seeds of Arceuthobium spp. falling onto the plumage of birds, or the fur of animals, tend to stick and may be dispersed for long distances. About 7% of birds and mammals trapped carried seeds, rising to 22% during the 2-week period of maximum seed release (Hawksworth and Johnson, 1989).

Silvicultural Practices

Logging and movement of timber which has not been completely de-barked, can result in movement of complete plants of Arceuthobium and possible transfer of seeds and establishment of new infestations.

Accidental Introduction

Accidental introduction of Arceuthobium spp. into new areas or continents does not appear very likely. Seeds are short-lived, and unlikely to reach a host tree under circumstances in which they could develop. Conifer plants could carry living mistletoe plants, especially in the prolonged endophytic stage before the external plant develops, but young plants, as normally traded, are not very likely to be infected. Mistletoes could be carried on cut branches, including Christmas trees, and possibly on logs with bark (though mistletoes normally occur on the branches of trees, not on trunks). It is unlikely that mistletoes borne on cut, dead plants present any risk of transmission. Accordingly, introduction can be prevented relatively easily. The prohibition of import of plants for planting of the main host genera (as established, for example, in the phytosanitary regulations of the European Union) blocks the only really dangerous pathway.

Intentional Introduction

Intentional introduction seems extremely unlikely, other than for research.

Plant Trade

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Plant parts liable to carry the pest in trade/transport	Pest stages	Borne internally	Borne externally	Visibility of pest or symptoms
Bark	whole plants
Stems (above ground)/Shoots/Trunks/Branches	whole plants

Plant parts not known to carry the pest in trade/transport
Bulbs/Tubers/Corms/Rhizomes
Flowers/Inflorescences/Cones/Calyx
Fruits (inc. pods)
Growing medium accompanying plants
Leaves
Roots
Seedlings/Micropropagated plants
True seeds (inc. grain)

Impact Summary

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Category	Impact
Animal/plant collections	None
Animal/plant products	None
Biodiversity (generally)	None
Crop production	None
Environment (generally)	None
Fisheries / aquaculture	None
Forestry production	Negative
Human health	None
Livestock production	None
Native fauna	None
Native flora	None
Rare/protected species	None
Tourism	None
Trade/international relations	Negative
Transport/travel	None

Impact

Top of page Arceuthobium species as a whole are regarded as some of the most serious of all pests/diseases of North American forests. Dwarf mistletoes are much more damaging to their hosts than the 'green' mistletoes in both Loranthaceae and Viscaceae. Having little photosynthetic capacity, they draw more heavily on host carbohydrate, and furthermore interfere with photosynthate translocation to the roots. The mistletoe has a girdling effect, resulting in an accumulation of photosynthate above the site of infection. Apparently carbohydrates are withheld from the roots in quantities sufficient to cause the characteristic decline of the tree (Rediske and Shea, 1961; Hawksworth and Wiens, 1996). There are also severe growth-regulatory effects resulting from cytokinin production at the point of infection and the redirection of host photosynthate into the resulting witches broom growths. These distort and suppress growth of branches and even the main trunk. Wood quality is further affected as a result of swellings, witches' brooms and knots, and structural weakening associated with shortened, distorted tracheids.

A. tsugense is a severe pathogen of Tsuga mertensiana and T. heterophylla in western North America.

Risk and Impact Factors

Top of page Invasiveness

Invasive in its native range
Highly mobile locally
Has high reproductive potential

Impact outcomes

Negatively impacts agriculture

Impact mechanisms

Competition - monopolizing resources

Likelihood of entry/control

Difficult/costly to control

Similarities to Other Species/Conditions

Top of page A. tsugense resembles A. laricis, which occasionally occurs on T. mertensiana.

Prevention and Control

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Due to the variable regulations around (de)registration of pesticides, your national list of registered pesticides or relevant authority should be consulted to determine which products are legally allowed for use in your country when considering chemical control. Pesticides should always be used in a lawful manner, consistent with the product's label.

The following methods for controlling Arceuthobium spp. may have utility for the management of A. tsugense.
Cultural Control

In the absence of any simple direct means of control of dwarf mistletoes, and the vast areas of forest involved, cultural management is virtually the only approach to the problem, the techniques varying according to the type of stand in which the problem occurs. Management options listed by Hawksworth and Johnson (1989) include:

- Survey

- Use RMYLD model to predict yields (Edminster, 1978; Hawksworth, 1978)

- Favour or plant resistant tree species

- Prune infected branches and witches' brooms

- Destroy the whole stand (including the use of fire) and regenerate

- Fell non-merchantable infected trees

- Sanitation thin

- Harvest and regenerate the stand

- Do nothing.

Hawksworth and Johnson (1989) also refer to mechanisms to help prevent infection, including the use of natural or man-made barriers (roads, streams, strips of non-susceptible forest) to reduce (re)invasion from adjacent infested stands; and removing infected trees before re-planting/regeneration.

Detailed surveys are an essential ingredient of successful control programmes and the 6-class rating system (Hawksworth, 1977) is widely accepted as a standard. This involves a 0-6 score based on 0, 1 or 2 for each third (lower, middle, upper) of the tree; 0 for no infection, 1 for light infection (less than half the branches affected) or 2 for heavy infection (more than half infected).

In recently harvested, regenerating stands, the emphasis is on the complete removal of any infected trees over 2 m, regardless of commercial value, both within the stand, and along borders to a distance of 18 m, before the regeneration is 1 m high.

In pre-commercial stands in which surveys show less than 40% infected trees, it should be economic to practice selective thinning to remove all those infected. Above 40% this is unlikely to be economic. Severely infested stands may best be harvested early and regenerated, but decisions may require use of available models to help devise the most economic option. Some of the available models are described by Muir and Geils (2002).

Dwarf mistletoes may contribute in various ways to biodiversity - by creating openings in the forest following tree death, by providing nesting sites in the 'brooms' and by providing food for a range of vertebrates and invertebrates. There can therefore be some conflict between the requirements of forest exploitation, and environmental concerns.

Mechanical Control

Pruning may be appropriate as a means of reducing damage to individual trees, but more generally to reduce the source of infection for surrounding trees. The practicality, however, is that it will only be feasible in particular amenity and recreation areas.

Clear-felling (with or without fire) is appropriate where a stand is so severely infested that it needs to be abandoned and regenerated or re-planted.

Chemical Control

The only chemical approved for use against dwarf mistletoes is the ethylene-releasing growth regulator, ethephon, which can cause abscission of the shoots and delay fresh seeding for 2-4 years, but there is eventual re-growth from the endophyte. It is difficult to achieve good coverage in larger trees from the ground, whereas applications from the air fail to penetrate the canopy adequately. The treatment is therefore of interest mainly for high-value amenity trees.

Biological Control

Glomerella cingulata is being developed as a biocontrol agent for use on A. americanum and A. tsugense (Geils et al., 2002) and has shown promise in field trials. Work is also in progress on two other pathogens, Caliciopsis arceuthobii and Nectria neomacrospora. There is no known method for biological control of A. tsugense.

Integrated Control

Hawksworth and Johnson (1989) emphasise the importance of integrating dwarf mistletoe control with measures to reduce damage from the mountain pine beetle (Dendroctonus ponderosae).

References

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Deeks SJ; Shamoun SF; Punja ZK, 2001. In vitro germination and development of western hemlock dwarf mistletoe. Plant Cell, Tissue and Organ Culture, 66(2):97-105; 45 ref.

Edminster CB, 1978. RMYLD: computation of yield tables for even-aged and two-storied stands. Research Paper RM-199, Fort Collins, USA: United States Department of Agriculture Forest Service.

EPPO, 2014. PQR database. Paris, France: European and Mediterranean Plant Protection Organization. http://www.eppo.int/DATABASES/pqr/pqr.htm

Geils BW; Tovar JC; Moody B; (technical coordinators), 2002. Mistletoes of North American Conifers. General Technical Report RMRS-GTR-98. Ogden, USA: United States Department of Agriculture Forest Service.

Hawksworth FG, 1958. Rate of spread and intensification of dwarf mistletoe in young Lodgepole Pine stands. Journal of Forestry, 56:404-407.

Hawksworth FG, 1977. The 6-class dwarf mistletoe rating system. USDA Forest Service General Technical Report, Rocky Mountain Forest and Range Experiment Station, No. RM-48:7 pp.

Hawksworth FG, 1978. Intermediate cuttings in mistletoe-infested lodgepole pine and southwestern ponderosa pine stands. General Technical Report, Pacific Southwest Forest and Range Experiment Station, No. PSW-31:86-92

Hawksworth FG; Johnson DW, 1989. Biology and management of dwarf mistletoe in lodgepole pine in the Rocky Mountains. General Technical Report - Rocky Mountain Forest and Range Experiment Station, USDA Forest Service, No. RM-169:ii + 38 pp.

Hawksworth FG; Wiens D, 1996. Dwarf Mistletoes: Biology, Pathology, and Systematics. Agriculture Handbook 709. Washington DC, USA: United States Department of Agriculture Forest Service.

Hinds TE; Hawksworth FG, 1965. Seed dispersal velocity in four dwarf mistletoes. Science, 148:517-519.

Kope HH; Shamoun SF; Oleskevich C, 1997. First report of Colletotrichum gloeosporioides on Arceuthobium tsugense subsp. tsugense in Canada. Plant Disease, 81(9):1095; 2 ref.

Muir JA; Geils BW, 2002. Management strategies for dwarf mistletoe: silviculture. General Technical Report - Rocky Mountain Research Station, USDA Forest Service, No. RMRS-GTR-98:83-94.

Nickrent DL; Stell AL, 1990. Electrophoretic evidence for genetic differentiation in two host races of hemlock dwarf mistletoe (Arceuthobium tsugense). Biochemical Systematics and Ecology, 18(4):267-280

Rediske JH; Shea KR, 1961. The production and translocation of photosynthate in dwarf mistletoe and Lodgepole Pine. American Journal of Botany, 48:447-452.

Smith RB; Funk A, 1980. Assessing Nectria macrospora as a biological control agent for hemlock dwarf mistletoe. Bi-monthly Research Notes, 36(3)

Sproule A, 1996. Impact of dwarf mistletoe on some aspects of the reproductive biology of jack pine. Forestry Chronicle, 72(3):303-306; 11 ref.

Distribution References

CABI, Undated. CABI Compendium: Status as determined by CABI editor. Wallingford, UK: CABI

EPPO, 2020. EPPO Global database. In: EPPO Global database, Paris, France: EPPO.

Hawksworth FG, Wiens D, 1996. Dwarf Mistletoes: Biology, Pathology, and Systematics. In: Agriculture Handbook 709, Washington DC, USA: United States Department of Agriculture Forest Service.

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Datasheet

Arceuthobium tsugense (hemlock dwarf mistletoe)

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