Arceuthobium abietinum
(fir dwarf mistletoe)

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.

Parasitic
Perennial
Seed propagated

A. abietinum is widely distributed from Washington through Oregon and California, to Chihuahua State in northern Mexico (f.sp. magnificae only in California and south-west Oregon). There are isolated records elsewhere in the western part of the USA.

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.

A. abietinum is limited to forests in which its specific hosts are present in substantial numbers.

When growing nearby parasitized Abies spp. other conifers will occasionally be attacked by A .abietinum, such as Picea breweriana and rarely, certain Pinus spp. (Pinus lambertiana, P. monticola and P. contorta).

Abies amabilis and A. procera have very rarely or never been recorded as hosts.

Genetics

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

Physiology and Phenology

Like other Arceuthobium spp., A. abietinum 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. are dioecious. Pollination appears to be predominantly due to insects (especially ants and flies) but may also occur by wind (Hawksworth and Wiens, 1996). In A. abietinum f.sp. concoloris, peak anthesis usually occurs from mid-July to mid-August, with extremes from early July to late September. Fruits usually mature in September or October, with extremes from late August to early November. In A. abietinum f.sp. magnificae, peak anthesis is usually from early August to mid-September, with extremes from mid-July to late September. Fruits mature from early September to late October. The maturation period for both f.sp. averages 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.

Associations

Canker due to Cytospora abietis is common on firs parasitized by A. abietinum (Hawksworth and Wiens, 1996).

Major fungal parasites of Arceuthobium spp. in North America that occur on A. abietinum are Caliciopsis arceuthobii and Colletotrichum gloeosporioides (Hawksworth and Wiens, 1996).

The brown felt blight fungus (Herpotrichia juniperi) has been found infecting A. abietinum as well as its Abies host. Its potential as a biological control agent with G. cingulata has been considered (Scharpf, 1986).

A dwarf mistletoe on Abies is most likely to be A. abietinum. A. douglasii, A.laricis and A. tsugense (q.v.) occur on Abies, but are much smaller and are different in colour. In central Mexico, another species (A. abietis-religiosa) is larger, olive-green and occurs on A. religiosa.

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.

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 do 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 of some Arceuthobium spp. 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, while applications from the air fail to penetrate the canopy adequately. The treatment is therefore of interest mainly for high-value amenity trees. It is not clear whether ethephon is practical or effective for the control of A. abietinum.
Biological Control

Colletotrichum gloeosporioides [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.

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).