Forest and Wildlife Extension

Forest Health Notes

A Series for the Non-Industrial Private Forest Landowner

Dwarf Mistletoe
(Arceuthobium spp.)

Hosts: Ponderosa pine, lodgepole pine, western larch, Douglas-fir, western hemlock,
mountain hemlock, rarely western white pine and spruce.

Quick ID:

• Pronounced "witches' brooms"
  
• Spindle-shaped swellings on branches
  
• Swollen areas on trunks that may be twice the normal diameter of the tree 1/2- to 6 inch
  
• Green shoots protruding from swollen areas on branches and trunks

Field Identification

Tree: In response to branch infections, trees often produce "witches' brooms," abnormal proliferations of many small twigs which appear as a mass of twigs and foliage. These vary in appearance from tight clumps to large loose fans. The dwarf mistletoe species that infect Douglas-fir, ponderosa pine, lodgepole pine, western larch, and hemlock cause large witches' brooms. Dwarf mistletoe infections on branches have a spindle-shaped, swollen appearance. On trunks,
infections may cause the tree to swell to twice its original diameter.

Plant: Shoots of the plant protrude from the swellings on branches and trunks. These are leafless and vary in color and size according to species; they have different male and female forms on the same tree. Douglas-fir dwarf mistletoe has 1/4- to 1/2-inch olive green shoots, while ponderosa pine dwarf mistletoe has 3- to 8-inch olive green to yellow shoots. Lodgepole pine dwarf mistletoe
has 2.5-inch yellow to olive-green shoots and western larch dwarf mistletoe has shoots that are dark purple and 1.5 to 4 inches long. Hemlock dwarf mistletoe has 2-imch shoots that are either green or red, and true fir dwarf mistletoe has shoots that are yellow-green and 3 to 8 inches long.

May be confused with: Elytroderma needle blight on ponderosa pine, simulation brooms caused by release of suppressed trees on hemlock or lodgepole pine, brooms caused by viruses on Douglas-fir, fir broom rust.

Disease cycle: Dwarf mistletoe is probably the most damaging disease of larch and lodgepole pine in Washington, causing growth loss, wood quality reduction and tree killing. Dwarf mistletoe is a plant, but is entirely dependent on its host tree. Death of the tree also means death of the mistletoe, so mistletoes tend to coexist with their hosts. Dwarf mistletoes are fairly host specific; there is limited crossover from one species of tree to another. Seeds are sticky and are forcibly shot from shoots growing on swellings. These seeds may travel up to 100 feet depending on the species and wind. When they land on the proper host, they sprout in the spring and penetrate the thin bark, forming a new infection. Spread is fastest in multi-storied stands that are somewhat
open; in single-story stands the spread averages 1-2 feet per year. Birds may be responsible for long-distance spread by carrying seeds stuck to their feet or feathers. Trunk infections cause swelling and cracking that permits rot fungi to be introduced into the heartwood. Trees that are less than 3 feet in height or 10 years in age generally escape infection.

Predisposing agents: The classic problems with dwarf mistletoe have occurred as a result of a multi-storied forest structure, with older trees raining seed down on younger ones. This is problematic in Eastern Washington as there are many Douglas-fir/grand fir or pure Douglas-fir stands that have this type of structure. These stands are typically regenerated by cutting the overstory and allowing the infected understory to grow up to form the new crop, thus perpetuating the disease. Because of the dwarf mistletoe's host specificity, pure stands or stands with
only a few species have more problems than mixed stands of several species.

Impact: Dwarf mistletoe infests 42% of stands of Douglas-fir on the East Side. While dwarf mistletoe on Douglas-fir is unusual on the west side of the Cascades, it is a problem on hemlock. In the Pacific Northwest, it infests 47% of western larch stands, 42% of lodgepole pine stands, 21% of hemlock stands, and 26% of ponderosa pine stands. True fir dwarf mistletoe infestations are uncommon in Washington. The major impact of dwarf mistletoe infestation is in growth loss. An estimated 148 million cubic feet are lost annually in all species to dwarf mistletoe in Washington and Oregon, and 40 million cubic feet are lost in Douglas-fir in the Inland Empire.

Management: Choice of silvicultural method depends upon the management goals for the stand, keeping in mind the stage of the cutting cycle and the damage being inflicted by the disease. With young stands, the objective is generally to protect them from infection. With middle-aged stands, efforts are usually directed towards reducing infection levels and halting spread. When the stand has reached harvest age, the objective is to eliminate mistletoe and thus prevent the subsequent crop from be coming infected. When evaluating the condition of the stand, determine two things: whether the stand is made up of more or less than 50% of the host species, and whether more or less than 50% of the trees of the host species are infected. As the infection may be clumped inside a stand, sometimes it is easier to subdivide the stand and deal with small areas separately.
These treatments would be appropriate for dealing with dwarf mistletoe infections in all species:

• Lightly sanitize. This is a good option if the stand is made up of less than 50% of the host species, and less than half of the host species is infected. Remove the diseased individuals to halt the spread. This can be done by combining sanitizing with a thinning or selective harvesting operation. It may be necessary to return a couple of years later to remove trees that were not noticeably infected until thinning allowed more light. into the stand and stimulated shoot growth.
  
• Stand conversion. If the stand is made up of less than 50% of the host species, but more than half of them are infected, it would be best to convert the stand to another species by cutting all the trees of the infected species, including the regeneration. This may be done in a thinning or partial harvest. Choosing this option depends in part on stocking levels and stand age, as cutting all the trees of one species may leave the stand unacceptably stocked; in that case it may be better to cut the most heavily infected individuals and permit the lightly infected to stay until rotation age.
  
• Sanitize. If the stand is made up of more than 50% of the host species, and less than half of the host species are infected, a thinning/sanitation cut of diseased individuals would be appropriate in a well-stocked or overstocked stand. In an understocked stand it may be better to wait until a commercial harvest may be made, especially if the stand is nearing rotation age.
• Clearcut. If the stand is made up of more than 50% of the host species, and more than half of them are infected, and the stand is still quite young (less than 20 years,) clearcutting and starting over may be the best option. If the stand is middle-aged, it may be best to cut the most seriously infected trees and leave the more lightly infected until rotation age.
• Do nothing. If the host species is more than 50% of the stand, and more than half of the trees are infected, a sanitation cut of diseased individuals may leave stocking levels too low. In that case, if the trees are nearly merchantable, it is better to do nothing.

When regenerating a stand, a seed tree or shelterwood cut may be used if the overstory is removed before the young trees are greater than 3 feet in height or have reached 10 years of age, as they will probably escape infection. Delaying removal of the overstory, however, can seriously jeopardize the new stand. It is beneficial to establish a single-storied, mixed-species stand when regenerating as that sharply slows the spread of mistletoe from surrounding stands, and will also help manage some problems such as root diseases and damaging insects. Combining a seed tree cut with planting may be an inexpensive way to get a mixed-species stand. Lastly, if the stand to be regenerated is surrounded by dwarf mistletoe infected trees, and the species desired is the same species as the infected trees, a buffer strip wider than the projectile range of the seeds should be planted with another species around the perimeter of the new stand.

Adapted from J.W. Schwandt, Dwarf mistletoe management strategies for inland Douglas-fir and Grand fir types. In: Silvicultural management strategies for pests of the interior Douglas-fir and Grand fir forest types, proceedings of a symposium held February 14-16, 1984 and available from WSU Cooperative Extension.

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Forest Health Notes were prepared by Donna Dekker-Robertson, Peter Griessmann, Dave Baumgartner, and Don Hanley, Washington State University Extension. The assistance of Robert L. Edmonds and Robert I. Gara, University of Washington College of Forest Resources, is gratefully acknowledged.

Insect or disease treatments should always be part of an overall forest management planning process. It's a good idea to have a forest management plan for your property. This tool will help you prevent problems and keep your forest healthy. For information on forest stewardship educational programs, contact your local WSU Extension office. For information on technical assistance and financial incentive programs, contact the Washington Department of Natural Resources. The Natural Resources Conservation Service will assist landowners in developing conservation plans.