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

 
     
 

ANNOSUS ROOT DISEASE
(Heterobasidion annosum, Fomes annosus)

Hosts: All coniferous species.
Highly susceptible: Grand fir, mountain hemlock, western hemlock
Moderately susceptible: Subalpine fir, western white pine, ponderosa pine, lodgepole pine
Less susceptible: Douglas-fir, larch, spruce, western redcedar, hardwoods

Quick ID:

  • Crown yellowing and thinning
  • Distress crops of cones
  • Trees killed in disease centers
  • Windthrown trees
  • Conks on declining live trees, dead trees, and stumps
  • Annual rings separate with pits on one side of sheets

Annosus Root DiseaseField Identification

Tree & Stand: Symptoms vary by species. In hemlock, crown symptoms are uncommon and trees often look healthy until they are windthrown. In other species such as pine, affected trees
show marked reduction in height and diameter growth. The crown thins and yellows, and may frequently produce a distress crop of cones. Resinous hosts, particularly pines, may have
pitch soaking of root wood, and crown symptoms are more apparent than in nonresinous species such as true firs. Decay appears initially as a reddish-brown stain with an irregular margin
in the interior of butts and roots. In the roots, decay has several symptoms: the bark separates easily from the wood, the wood is streaked with darker brown lines and the surface of the inner
bark has numerous small silver to white flecks. Later, the roots may have small, shallow pockets of white fibers with black flecks. In the trunk, wood will often separate into annual rings with small oval pits on one side (as opposed to the pits of laminated root rot, which are on both sides). Advanced decay of the trunk is stringy with large white streaks, and black flecks are usually found in decayed wood. Disease centers where the disease has spread out over time from an initial infection are common. In the center, an old stump or empty area may be found. Trees within several yards to hundreds of feet (depending on the size and age of the infection) will be dead or dying, and trees further out will show early crown symptoms. Trees within 50 feet of the apparent edge of a disease center are very probably infected as well. Trees in the disease center may be windthrown, either living or dead.

Fungus: The most obvious sign of Annosus root rot is the fruiting body (conk), found inside old stumps or in the duff layer on the exterior of roots and root collars of infected or dead trees. The conk may take several forms. Small, button-like conks may be found on the wood beneath the bark of pine stumps and on the surface of small pine roots in the soil. On the root collar of infected trees, conks ranging from 1/2 to 2" in size may be found under the litter layer. Inside rotted stumps or on the underside of exposed roots of windthrown trees, conks tend to be large (up to 10" wide) and irregularly shaped. The conks, which are perennial, have a dark upper surface and a
creamy white lower surface covered with tiny pores.

May be confused with: Laminated root rot, Armillaria root rot.

Disease cycle: Annosus root rot is considered third in severity in the Pacific Northwest, after laminated root rot and Armillaria root rot. Researchers have identified at least two host-specific forms of the disease: an s-type that infects mostly spruce, firs, Douglas-fir, western redcedar, and hemlock, and a p-type that occurs mainly on pines but will infect the same species as the s-type, as well as brush and hardwood species. Unlike laminated or Armillaria root rot, Annosus spreads both by root contact and by spore infection. When the roots of a healthy individual touch the roots of a diseased individual, a new infection results. As well, windborne spores that are deposited on freshly-cut wood surfaces such as stumps or basal wounds germinate and infect the surface.
Annosus can live for 50 years in large stumps of resinous trees; in smaller stumps and nonresinous species survival time is shorter. Species susceptibility to attack varies. In some species such as hemlock, the host is rarely killed because of its ability to compartmentalize the decay in the butt log. In other species, notably pines and grand fir, the disease causes tree death. Trees are often killed standing rather than being windthrown.

Predisposing agents: Certain management practices such as harvesting old growth can create heavy inoculum levels. If the old stumps become infected subsequent regeneration will alsobecome infected. This creates a potential for future disease spread that is very serious, as windborne spores could be carried for some distance. Stumps less than 18" in diameter rarely become established infections. Partial or selective harvesting increases the likelihood of infection by creating new stumps and causing basal wounds from mechanical injury. Annosus is itself a predisposing factor for bark beetle (particularly fir engraver) attack, and is frequently found on
the same tree with Armillaria root rot or laminated root rot.

Impact: Total losses in board feet are not known, but Annosus causes damage by tree killing, butt rot, windthrow, and decrease in growth of affected trees. Losses due to Annosus are known to be increasing, probably as a result of intensified logging. Bark beetle epidemics mask the incidence of root disease by being the obvious killer, but many if not most trees killed by bark beetles are predisposed by root diseases. Additionally, bark beetle epidemics become established in diseased trees and spread out to healthy ones.

Management: It is important to search carefully for signs of other root diseases once Annosus has been diagnosed, as they often occur together. The presence of Armillaria or laminated root rot will affect choice of species for reforestation and silvicultural methods to follow. Unlike the other two major root diseases, there is a chemical control for Annosus. Borax powder spread 1/8" thick on a cut stump within two days of harvest will prevent colonization by Annosus spores. Stumps smaller than 18" in diameter need not be treated with borax as they rarely become sources of inoculum. Pine stumps should only be treated if they are within one mile of an infected pine stand.

Control of Annosus disease centers is best attempted at final harvest. A systematic survey of the stand can be used to identify the borders of the disease center; every infected tree should be marked low on the trunk so the mark remains after logging. A buffer strip 50 feet wide beyond the border of the disease center should be established. 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. Within the disease center and the buffer strip, every tree should be cut and stumps treated with borax unless they will be excavated. During the harvest, trees outside
the designated boundary should be checked for symptoms and the boundaries adjusted accordingly. Following harvest, stumps may be excavated if it is economical and practical considering species desired, terrain, and soil type. Otherwise, another, more resistant species may be planted. This is where detecting the presence of other root diseases is critical: Douglas-fir is not seriously damaged by Annosus, but is highly susceptible to Armillaria and laminated root rot. Ponderosa pine is moderately susceptible to Annosus, but in south central Washington is
highly susceptible to Armillaria. The best bet is blister rust resistant western white pine in stands with high mortality. If the previous stand was true fir, ponderosa pine or lodgepole pine may be planted as the strain of Annosus affecting true firs will not infect pines. Short rotations of 40 to 120 years in stands of grand fir and white fir will also minimize problems with Annosus, especially if the number of intermediate stand entries that may cause wounds is reduced. Thinning to increase stand vigor is recommended early in the rotation before the potential for wounding becomes significant; small stumps from thins do not need to be treated with borax. In mixed-species stands, favor species oher than true fir. Salvage logging should be minimized in true-fir stands as
it increases wounding.

 
                         
 
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.

Issued by Washington State University Cooperative Extension and the US Department of Agriculture in furtherance of the Acts of May 8 and June 30, 1914. Cooperative Extension programs and policies are consistent with federal and state laws and regulations on nondiscrimination regarding race, color, gender, national origin, religion, age disability, or sexual orientation. Evidence of noncompliance may be reported through your local Cooperative Extension office. Trade names have been used to simplify information; no endorsement is intended.

 
                         
 
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