Dwarf Mistletoe Damage

Consider the two Pseudotsuga menziesii (Douglas-fir) infected by Arceuthobium douglasii in the image above. The tree at left is all brooms, stunted, and deformed. The tree at right is dying. The large brooms in live trees may provide preferred habitat for some animals, but neither tree will make a full-size snag that is most valuable to wildlife (see dwarf mistletoe ecology). For timber, the tree at left is largely worthless. The one at right could be merchantable, but only if it is harvested before or soon after it dies.

Effect of infection of Pinus contorta by dwarf mistletoe at an early age on volume (which integrates effects on height growth, diameter growth, and mortality). Data from ​[8]​.

In western North America, dwarf mistletoes are generally considered the most damaging type of forest disease. There are areas where root diseases are more important, but mistletoes are important in many forest types, on most major tree species, over vast areas. Important forms of damage include:

  1. Growth loss
  2. Mortality
  3. Predisposition to other mortality agents (e.g., bark beetles) and decay fungi (after old stem infections become cankerous)
  4. Dead tops
  5. Defect (stem swellings, cankers, other deformities)
  6. Reduced production and viability of seed

Growth impacts

An animation of the modeled impacts of Douglas-fir dwarf mistletoe shows the results of disease progression (Fig. 31 in ​[25]​). With moderate amounts of dwarf mistletoe at age 80, after 100 years the stand actually has lower basal area than it started with! This is a combined effect of growth and mortality effects. Young trees are particularly vulnerable to permanent stunting, deformity and mortality. But even mature trees have reduced lifespan and growth potential when severely infected.

Significant growth impact is seldom seen until at least the lower 1/3 to 1/2 of the crown is heavily infected. Light infection throughout the crown or heavy infection in just the lower crown usually cause little or no damage, except perhaps when large brooms are produced or stem infections cause problems. Brooms seem to have a special effect on growth. A large broom or several smaller ones can devastate the rest of the crown, leading to severe crown thinning.

Mortality impacts

Dwarf mistletoes increase host mortality, the effect becoming larger as infection severity increases (following table; data from ​[10, 11]​). Certain dwarf mistletoes are considered to be especially lethal ​[10]​. For example, 4 of the 5 dwarf mistletoes in the southern Rocky Mountains have that distinction.

Increase in 10-year mortality rate in stands infested with dwarf mistletoe

A. abietinumAbies concolor136101521
A. americanumPinus contorta245678
A. vaginatumP. ponderosa148152334
A. douglasiiPseudotsuga menziesii12491523
The percent of trees dying per decade was estimated for each stand. The value for uninfested control stands was subtracted from the corresponding infested stands, and the average of those values from multiple stands and studies is reported here.

Mortality can be strongly influenced by water relations. It is greater on dry than on wet sites ​[1]​ and also during or following a drought ​[6]​. Large trees survive longer than small trees (next table; data from ​[7]​), but even large, mature Pinus ponderosa can be killed by dwarf mistletoe ​[21]​.

Effect of dwarf mistletoe on host half-life

Initial DMRHalf-life for
4-9 in. DBH
Half-life for
> 9 in. DBH
Expected half-life (time in years for half the trees to die) of Pinus ponderosa infected with A. vaginatum ssp. cryptopodum at Grand Canyon National Park, USA.
ND: No decrease in longevity detected; half-life too long to estimate.

In P. contorta stands infested for 80 years, 15% of the standing basal area was killed by dwarf mistletoe (determined by subtracting standing mortality in similar but uninfested stands ​[8]​. However, most of the snags in that study were quite small.

Dwarf mistletoes frequently increase susceptibility to attack by bark beetles ​[10, 22]​. In Colorado, dwarf mistletoe increases susceptibility of P. ponderosa to mountain pine beetle (MPB) and western pine beetle ​[3, 4, 12, 15, 26]​. Infected P. ponderosa may also be selectively attacked by Ips spp. ​[13, 14]​. A similar effect is seen in P. edulis (piñon) ​[17, 24]​.

In the case of Pseudotsuga menziesii, dwarf mistletoe may have little effect on bark beetle susceptibility ​[5]​. However, such predisposition has been suggested for northern Idaho ​[23]​.

Lodgepole pine dwarf mistletoe generally decreases susceptibility of Pinus contorta to MPB ​[16, 20]​. However, some authors suggest little or no significant effect ​[9, 19]​. Decreased susceptibility, where it occurs, is probably due to smaller diameter and thinner phloem caused by dwarf mistletoe (Roe & Amman 1970). However, stem infections, which often have thicker bark than the rest of the tree, may be selectively attacked by MPB (McGregor 1978).

Economic impacts

It has been estimated that, in western USA, dwarf mistletoes cause direct, annual wood volume loss of 11.8 million m3 ​[2]​. This is roughly equivalent to 3.3 billion board feet. This estimate includes only commercial forest land on National Forests, and it doesn’t include defect and loss of wood quality. There is a great deal more dwarf mistletoe in non-timber lands and in other ownerships. That said, most losses are not realized, i.e., based on current management, much of it likely wouldn’t have been harvested even if healthy. In western Canada, a comparable loss estimate is 3.8 million m3 yr-1 ​[10]​.

Each year, dwarf mistletoes on National Forests of the western USA cause enough loss of wood production to build over 1 million small homes like thiis.

In the US study ​[2]​ the highest losses on an area basis were about 1.5 m3 ha-1 yr-1 for Pseudotsuga menziesii and Larix occidentalis in certain regions.

To put this in perspective, 11.8 million m3 is:

  • enough wood to build 1.1 million small, 3-bedroom houses each year (3000 bd ft each)!!!
  • over 13% of the 2016 harvest volume from the same western states (87.9 million m3) ​[18]​. This is an underestimate because the harvest data include non-USFS land.
  • From an ecological perspective, it is enough wood to make trees a lot larger and older if they were neither infected nor harvested.


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