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Forest Health Notes:
A Series for the Non-Industrial Private Forest Landowner

(Armillaria ostoyae)

Hosts: all coniferous species.
Highly susceptible: True firs, Douglas-fir, western hemlock
Moderately susceptible: spruces, western redcedar, lodgepole pine, western white pine
Least susceptible: ponderosa pine (except in south central Washington), western larch

Quick ID:

  • Resin flow from tree base
  • Crown thinning or changing color to yellow or red
  • Distress crop of cones
  • White mycelial fan under bark
  • Black rhizomorphs penetrating root surfaces
  • Honey-colored mushrooms near base of tree in fall
  • Affected trees often in groups or patches on the east side of the Cascades; usually killed singly on the west side.

Armillaria Root RotField Identification

Tree & Stand: The crowns of affected trees will begin to thin and change color, turning red, brown or yellowish. Resin blisters may be found on the stems and branches of young Douglas-firs.
Dead tissue may be found on the outside of the roots as decay progresses inwards; root centers often remain sound. Decayed wood may be stained grey to brown in the early stages, often
with a water-soaked appearance. Later, the wood becomes soft, yellowish, spongy and stringy, marked by frequent black zone lines. Occasionally there will be heavy resin flow from the tree
base. A distress crop of cones may be produced in trees of conebearing age. Large trees may be easily windthrown, with roots of down trees often broken crosswise near the base of the tree.

On the east side of the Cascades, Armillaria will commonly develop root rot centers, where the disease has spread out over time from an initial infection. In the center will be either an old stump or an empty area, or occasionally new regeneration. Trees within several yards to hundreds of feet (depending on the size and age of the infection) will be dead or dying, trees further out will show early crown symptoms, and trees beyond those will not yet show symptoms though they may be infected. These centers may be many acres in size. On the west side of the Cascades, trees are more commonly killed singly, and are mostly trees younger than 20 years of age that are under stress.

Fungus: In autumn, light brown to honey-colored gill fungi (mushrooms) with stalks 3 to 10 inches in length and caps 2 to 5 inches across may be found in clusters at the base of dead or dying trees and stumps. It is easier to diagnose this disease by stripping away a portion of bark at the root crown. A creamy white "fan" of leathery mycelia (fungal fibers) develops between the bark and wood of the infected host, and may be found beneath the bark of infected roots, root crowns and lower stems.

Rhizomorphs, black shoestring-like structures, also provide another reliable method of diagnosis. These may be found between the bark and the wood, on bark surfaces below the soil line, and in the litter and soil around the roots and root crown. They may be distinguished from rootlets in that they adhere to and penetrate roots as well as branching in a different manner.

May be confused with: Mottled rot (Pholiota limonella), which has similar mushrooms, but only if mycelial fans not present.

Disease cycle: Armillaria is the most common root rot in the Pacific Northwest. Armillaria is highly pathogenic, able to kill apparently healthy vigorous trees. Airborne spores don't seem to be very important in disease spread. Most commonly, trees are infected by rhizomorphs or by root contact with a diseased individual. Rhizomorphs grow out through the soil from an established infection and penetrate directly into the root surface of uninfected trees to spread the disease. When roots from a healthy tree touch an infected root or stump a new infection results.

Typically, young trees are quickly killed, while older trees may be able to block the progress of the fungus to the root collar and thus survive for many years, albeit in a weakened state (predisposed to windthrow, other diseases and bark beetle attack). Over time, rotted stumps accumulate in an area; these may retain viable inoculum for decades which may infect regeneration.

Predisposing agents: Stress generally predisposes trees to attack by assorted agents; a tree that is not receiving enough water, light, or soil nutrients, or one that has been exposed to temperature extremes, pollution, insect attack, disturbances from partial cutting, or other fungal diseases may have a reduced resistance to Armillaria attack. Armillaria occurs more frequently in dry areas, on less productive sites, and on sites disturbed by human activities (including fire suppression). Armillaria fre quently occurs with other root diseases such as laminated root rot and Annosus root disease. Armillaria itself is a predisposing factor for bark beetle attack. Young trees seem to be more susceptible than older trees of the same species.

Impact: Eastern Washington, with its dry forests, is among the areas where Armillaria occurs most frequently and severely. The problem has probably increased substantially in this century due to incorrect forest management practices such as selective logging of the ponderosa pine/western larch overstory and fire suppression, both of which favor regeneration of Douglas-fir and true fir which are more susceptible to Armillaria on most sites. In south central Washington, ponderosa pine is considered to be most susceptible and Douglas-fir appears tolerant, but in other areas Douglas-fir and true fir are the most susceptible. In Douglas-fir/true fir forests mortality begins shortly after regeneration and continues through the life of the stand, but in other species such as western redcedar, mountain hemlock, western larch, western white pine, ponderosa pine, and lodgepole pine, damage tends to diminish with stand age beyond 20-30 years. This is
probably more true on the west side rather than the east side of the Cascades.

Management: Armillaria often occurs with other root diseases and is a predisposing factor for bark beetle attack. For that reason, trees killed by bark beetles should be examined for signs of Armillaria or other root diseases, as that may affect choices about which species to replant and silvicultural methods to use. Armillaria can remain viable in stumps for 50 years. Chemical treatments have not been shown to be cost-effective. Nor is it possible to eradicate the fungus entirely. The most frequent and effective approach to managing root disease problems is to attempt to control them at final harvest by replanting site-suited tree species that are disease tolerant. In eastern Washington that typically means replacing Douglas-fir or true fir stands with ponderosa pine, western larch, western white pine, lodgepole pine, western redcedar, or spruce. Species susceptibility varies somewhat from location to location, so check what seems to be least affected on the site.

Prior to harvest, root disease centers should be marked by examining outlying trees for symptoms (e.g. mycelial fans, rhizomorphs). Infected trees should be marked low on the bole so the mark remains after harvest. It is suggested that disease centers should be mapped as well as marked to enable them to be tracked over time. Global Positioning Systems (GPS) may make mapping centers and tracking them over time easier, and that technology is rapidly becoming affordable. All trees in the disease center as well as uninfected trees within 50 feet should be cut. During the harvest, trees outside the designated boundary should be checked for symptoms and the boundaries adjusted accordingly. No tree from a highly susceptible species should be planted within 100 feet of a disease center.

Another more expensive alternative to changing species is to remove diseased stumps and trees from the site by pushing them out with a bulldozer. It is not necessary to burn the stumps as air drying will kill the fungus, and any small roots left underground will decay before they can reinfect the new seedlings. After stump removal, any species may be planted.

After planting, the most important control measure is to manage for reduced tree stress. Use silvicultural practices to regulate species composition, maintain biological diversity, reduce chances for insect pest buildup and increase host vigor. Thinning early in the rotation to promote species diversity is encouraged. Mixed-species forests are more resistant to insect defoliation, and also slow the spread of species-specific pests such as dwarf mistletoe, which are both predisposing agents for Armillaria.

Young stands should be checked for root disease symptoms at 15-20 years of age. Stands with four or more disease centers per acre are severely diseased and should not be precommercially thinned; instead, they should be either left untreated until rotation (at a younger age than normal), replanted with a more resistant species, or destroyed and replanted with Douglas-fir or other desirable species after stump removal. Stands with fewer than four disease centers per acre may be thinned by cutting infected trees as well as susceptible trees within 60 feet. However, any precommercial or commercial thinning program that removes existing ponderosa pine while leaving Douglas-fir and/or true fir is not recommended.

Selective harvesting of root-disease infested grand fir stands makes the problem worse by creating a fresh supply of stumps for colonization; for this reason, uneven-age management is not recommended (also because it intrinsically favors Douglas-fir and true fir stands). Salvage operations of grand fir and Douglas-fir in eastern Washington have apparently increased the amount of root disease losses. One major salvage every ten years seems to cause fewer problems than salvages every two to three years.

Adapted from Armillaria Root Disease, USDA Forest Service Agricultural Handbook No. 691

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

Department of Natural Resource Sciences
Washington State University Cooperative Extension
Pullman, Washington 99164-6410

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 Cooperative 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.

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