Arceuthobium laricis (larch dwarf mistletoe)

Summary of Invasiveness

Top of pageArceuthobium 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.

Notes on Taxonomy and Nomenclature

Top of pageA 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. laricis is a member of subgenus Vaginata, section Campylopoda, series Campylopoda.

Description

Top of pageA. laricis, like other Arceuthobium spp. is an parasites 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. A. laricis shoot are usually 4 to 6 cm high, dark purplish; flowering in summer or autumn; internodes 6 times as long as wide. Staminate flowers 2.7 mm across; perianth mostly 3-merous (sometimes 4-merous) not whorled, paler than the subtending bracts; pistillate flowers ca. 1 mm long, 1 mm across. Matute fruit 4.5 x 2.5 mm.

A polymerase chain reaction (PCR) method has been developed to detect A. laricis at the concealed endophyte phase of its development in L. occidentalis (Marler et al., 1999).

Plant Type

Top of pageParasitic
Perennial
Seed propagated

Distribution

Top of pageA. laricis's limited distribution in the northwest USA and southern British Columbia coincides with that of its host L. occidentalis.

Risk of Introduction

Top of pageThe 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 pageA. laricis is limited to forests in which its specific host, western larch, (Larix occidentalis) is present in substantial numbers.

Hosts/Species Affected

Top of pageA. laricis occasionally occurs on other conifers when they are growing nearby its principal host, Larix occidentalis. Tsuga mertensiana is frequently infected; Abies lasiocarpa, Pinus contorta and Pinus ponderosa are occasionally infected; A. grandis, P. monticola, P. albicaulis and Picea engelmannii are rarely infected. Mathiasen (1998) has confirmed the comparative susceptibility of these species. The exotic Larix europaea and Larix kaempferi have been shown to be susceptible on artificial inoculation. Exotic trees infected naturally by A. laricis include Pinus bankesiana, B. resinosa, P. sylvestris and Picea abies (Hawksworth and Wiens, 1996).

Growth Stages

Top of pageVegetative growing stage

Biology and Ecology

Top of pageGenetics

Chromosome number 2n = 28.

Physiology and Phenology

A. laricis, like other Arceuthobium spp., is an obligate parasite, 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 produce fruits annually, for at least 2 years, and often for 5 years or more (Hawksworth and Wiens, 1996).

Reproductive Biology

Arceuthobium spp., A. laricis is dioecious. Pollination appears to be predominantly due to insects (especially ants and flies) but may also occur by wind (Hawksworth and Wiens, 1996). Meoisis in A. laricis occurs in June. Peak anthesis takes place from mid-July to late August, with extremes from early July to early September (Hawksworth and Wiens,1996). Following fertilization of the 'ovule', fruits usually mature in September, with extremes from early August to early October. Maturation of the fruit takes about 13-14 months. 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 at least 10 m. 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.

Rain Regime

Top of pageUniform

Notes on Natural Enemies

Top of pageVarious insects and fungi have been recorded attacking Arceuthobium spp. but none is noted to have a particular importance on A. laricis. Caliciopsis arceuthobii and Glomerella cingulata, major parasites of Arceuthobium spp. in North America, have been found on A. laricis in the USA (Hawksworth and Wiens, 1996).

Means of Movement and Dispersal

Top of pageNatural Dispersal (Non-Biotic)

In common with other Arceuthobium sp. natural dispersal of A. laricis is by the explosive fruits which can expel the seeds for some distance from the parent plant (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 Arceuthobium seeds, rising to 22% during the 2-week period of maximum seed release (Hawksworth and Johnson, 1989).

Agricultural 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 seems 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.

Impact

Top of pageArceuthobium 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. laricis is a severe pathogen of Larix occidentalis in western North America. Infection usually results in the formation of heavy, compact brooms. Because larch branches are brittle, larger brooms are readily broken off. Surveys in the 1950s show that about two thirds of L. occidentalis stands were infested in the Coeur d'Alene National Forest, Idaho, USA, and the Kootenai National Forest, Montana, USA (Graham 1959a, 1959b). In the Colville National Forest and adjacent private lands in northeastern Washington, infestation was 86% (Graham and Frazier, 1962).

Similarities to Other Species/Conditions

Top of pageA. laricis resembles A. tsugense but the latter is rarely found on L. occidentalis.

Prevention and Control

Top of pageCultural 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) for the control of Arceuthobium spp. 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. Tests of ethephon for the control of A. laricis on Larix occidentalis in Oregon have given promising results: abscission rates of 90-100% occurred when there was thorough spray coverage (Hawksworth and Wiens, 1996). However, 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

No insects or fungi have been developed as biological control agents for A. laricis.

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

Top of page

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.

Graham DP, 1959. Dwarf mistletoes survey in Coeur d'Alene National Forest. Research Note 68. Ogden, UT, USA: US Department of Agriculture, Forest Service, Intermountain Forest and Range Experiment Station.

Graham DP, 1959. Dwarf mistletoes survey in Kootenai National Forest. Research Note 67. Ogden, UT, USA: US Department of Agriculture, Forest Service, Intermountain Forest and Range Experiment Station.

Graham DP, Frazier WE, 1962. Dwarf mistletoes survey in northeastern Washington. Research Note 103. Ogden, UT, USA: US Department of Agriculture, Forest Service, Intermountain Forest and Range Experiment Station.

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.

Marler M, Pedersen D, Mitchell-Olds T, Callaway RM, 1999. A polymerase chain reaction method for detecting dwarf mistletoe infection in douglas-fir and western larch. Canadian Journal of Forest Research, 29(9):1317-1321; 19 ref.

Mathiasen RL, 1998. Comparative susceptibility of conifers to larch dwarf mistletoe in the Pacific Northwest. Forest Science, 44(4):559-568; 41 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.

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

Distribution Maps

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