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Plantwise Technical Factsheet

eastern dwarf mistletoe (Arceuthobium pusillum)

Host plants / species affected
Abies balsamea (balsam fir)
Larix laricina (American larch)
Picea abies (common spruce)
Picea glauca (white spruce)
Picea mariana (black spruce)
Picea pungens (blue spruce)
Picea rubens (red spruce)
Pinus banksiana (jack pine)
Pinus resinosa (red pine)
Pinus strobus (eastern white pine)
Arceuthobium spp. are obligate 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, forming a systemic infection of whole branch systems. A. pusillum has some of the smallest shoots of any Arceuthobium species, only about 1 cm high with a maximum of 3 cm height, virtually unbranched. They appear 2-4 years after the original infection and are pale yellow-green, or male shoots may be reddish. Basal diameter of the shoots is about 1 mm. Third internode 1-4 mm long, about 1 mm thick, length to width ratio 1.9:1. Plants are dioecious. Staminate flowers about 2 mm across, perianth mostly 3-merous (occasionally 2- or 4-merous), segments ca. 0.8 mm long, 0.7 mm wide. Anther diameter 0.4 mm centred 0.5 mm from the tip of the perianth segment. Pollen polar diameter about 22 µm, equatorial diameter 26 µm, spine height 2.2 µm. Mature fruit, green, ca. 3.0 mm long by 1.5 mm wide, proximal portion ca. 2 mm long. Seeds 2.0 by 0.9 mm.
Prevention and control
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 for dealing with this species. The techniques vary according to the type of stand in which the problem occurs. Silvicultural treatments considered by Muir and Geils (2002) for cultural control of Arceuthobium species in general include:

- Harvest, retention, and regeneration by clear-felling (even-aged silviculture), or selective harvesting to establish and maintain uneven or all-aged stand stuctures

- Design and layout of harvest and treatment blocks

- Site preparation and vegetation management by brushing, prescribed burning, and other methods

- Planting or retaining residual and advanced regeneration

- Thinning and sanitation

- Pruning brooms and infected branches

Silvicultural guides with specific recommendations for control of A. pusillum in Picea mariana include Johnson (1977) and Ostry and Nicholls (1979).

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 branches affected) or 2 for heavy infection (more than half infected).

The general aim of management is to reduce the risk of further spread of the mistletoe by removing sources of infection and/or creating barriers to its movement. In recently harvested, regenerating stands, the emphasis may be 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).

In many cases the presence of dwarf mistletoe may pose no serious threat to productivity. Where wildlife objectives take precedence, retention of some dwarf mistletoe may even be desired. It 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.

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

Due to the variable regulations around (de-)registration of pesticides, we are for the moment not including any specific chemical control recommendations. For further information, we recommend you visit the following resources:

Biological Control

The use of the fungus Caliciopsis arceuthobii for biological control of A. pusillum has been considered, but potential is apparently limited by extreme variation in infection rates from one season to another.
Arceuthobium species as a whole are regarded as some of the most serious of all pests/diseases of N. American forests. Dwarf mistletoes are very 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.

In spite of its small size, A. pusillum is a very damaging parasite. Mortality is severe in Picea mariana along the coast of Maine, USA, and it is considered the most serious disease agent of P. mariana in the Great Lakes region of the USA (Geils et al., 2002). In Manitoba, Canada, infection of Picea glauca by A. pusillum ranged from 25 to 100%. The presence of mistletoe reduced the annual radial increment from 0.67 cm to 0.04-0.14 cm over a period of 5 years. Seedlings suffered 20% mortality over a 2-year period and 25% of the survivors were infected. The annual height growth of infected seedlings was reduced by 40% (French et al., 1981). In New Hampshire, USA, P. pusillum causes growth reduction and mortality of Picea rubens. It also reduces timber quality by causing marked trunk swellings (Hawksworth and Shigo, 1980).
Summary of invasiveness
A. pusillum does not spread rapidly and cannot be considered highly invasive. It does, however, constitute a serious threat as a result of its ability to build up gradually over the life of a forest and cause severe damaging effects on a number of important forest species. Its 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 the known hosts, planted as exotics in other countries, but there is also a possibility of spread to related species that are not known to be hosts.
Related treatment support
External factsheets
Maine Forest Service Insect and Disease Factsheets, Maine Department of Conservation, English language
APSnet Plant Disease Lessons, APSnet, 2006, English language
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