Dwarf mistletoes are parasites of native conifer forests that can cause severe damage.

 Dwarf Mistletoe: Parasitic Plants

Forty-two species of dwarf mistletoe are known worldwide; five species are found in Colorado’s forests. Most dwarf mistletoes are native to western North America, from Alaska south through the western United States, Mexico and Central America.

Dwarf mistletoe, a common problem in Colorado forests, predominantly affects ponderosa and lodgepole pines, although they can attack Douglas-fir, piñon, limber and bristlecone pines.

Dwarf mistletoes are parasites of native conifer forests that can cause severe damage. These plants tend to confine their attacks to one species of tree, with only occasional attacks on other species.

Dwarf mistletoe infections can retard growth and reduce seed production and wood quality; heavy, long-term infections can kill trees.

Some dwarf mistletoe species induce abnormal tree growth at the point of infection, and produce a structure known as a witches’ broom, which disrupts the typical branching structure.

Dwarf mistletoes are small, leafless parasitic flowering plants that kill by slowly robbing the tree of food and water.

Symptoms of Infestation

The first symptom of dwarf mistletoe infection is a slight swelling of the bark at the infection site. The parasite is identifiable when the yellow to green or brownish-green segmented shoots protrude from the infected part of the tree, which form about 2-3 years after the infection. Because dwarf mistletoes are slow killers, long-term management options are feasible.

 Dwarf Mistletoe Species of Colorado’s Forests

      
Dwarf Mistletoe SpeciesLodgepole Pine Dwarf Mistletoe
Arceuthobium americanum
Douglas-Fir Dwarf Mistletoe
Arceuthobium douglasii
Southwestern Dwarf Mistletoe
Arceuthobium vaginatum cryptopodum
Limber Pine Dwarf Mistletoe
Arceuthobium cyanocarpum
Piñon Dwarf Mistletoe
Arceuthobium divaricatum
Range in ColoradoWidespread; roughly 50% of lodgepole pine in Colorado have some degree of infectionSouthwestern Colorado and the Western SlopeSpotty distribution throughout host range in ColoradoRocky Mountain range throughout Colorado, primarily along the Front RangeThroughout host range in Four Corners region and western Colorado
Primary Host SpeciesLodgepole pineDouglas-firPonderosa pineLimber pine; Rocky Mountain bristlecone pinePiñon pine
Plant CharacteristicsOlive-green to yellow, 2-4 inches longOlive-green, less than 1 inch longOrange, reddish-brown to black, 4-6 inches longYellow-green, 1-3 inches longOlive-green to brown, 3-5 inches long
Host ResponseConspicuous witches' brooms, heavily infested crowns appear thinFan-shaped and spherical witches' broomsSmall but conspicuous brooms on branchesSmall and compact witches' broomsSmall and compact witches' brooms
ImpactInfestations can significantly reduce growth by 80-90% and predispose trees to other insects and diseasesWitches' brooms may weigh in excess of several hundred pounds and cause severely swollen branchesReduced growth, foliage discoloration and dieback reduce vigor of host treeConsidered second most important disease of high-elevation five-needle pines, causing severe mortalityCan cause increased mortality of piñon pines and significant deformation of host

Source: 2010 Report on the Health of Colorado’s Forests

Management Options

  • Pruning and removing infected trees is the best management measure available to reduce or eliminate dwarf mistletoe infestations.
  • Plant resistant trees under infected ones to replenish the forest after infected ones are removed.
  • If planting the same species as the infected tree, use approved sprays to reduce the risk of infestation. (Always carefully read and follow all label instructions before applying insecticides.)

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