Arceuthobium campylopodum (western dwarf misletoe)

Resistance to A. campylopodum is known in Pinus ponderosa and P. jeffreyi (Scharpf, 1984).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. General management options for dwarf mistletoes 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.

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.

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. Preliminary tests of ethephon for use against A. campylopodum on P. jeffreyii in California has given promising results, yielding shoot abscission rates of 90-100% when coverage is thorough. However, rapid resprouting from the endophytic system in A. campylopodum may limit its effectiveness (Parks and Hoffman, 1991). It is difficult to achieve good spray coverage of ethephon 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.

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.

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