Forest and Wildlife Extension

Forest Health Notes

A Series for the Non-Industrial Private Forest Landowner

Laminated Root Rot, Yellow Ring Rot
(Phellinus (Poria) weirii)

Hosts: Most conifers
Highly susceptible: Douglas-fir, mountain hemlock, western hemlock, grand and Pacific silver fir
Moderately susceptible: spruces, larch, noble and subalpine fir, western redcedar
Tolerant: lodgepole pine, western white pine
Resistant: ponderosa pine
Immune: Hardwoods

Quick ID:

• Crown yellowing and thinning
  
• Distress crop of cones, especially in Douglas-fir
  
• Frequent windthrow with "root balls"
  
• Buff colored mycelia may occur on the outside of roots.
  
• Outer heartwood stained red-brown
  
• Annual rings separate
  
• Reddish brown fungal hairs between sheets of decayed wood (use hand lens to identify)
  
• Affected trees often in groups or patches

Field Identification

Tree & Stand: Affected trees show marked reduction in height and diameter growth. The crown thins and yellows, and may frequently produce a distress crop of cones. Trees are commonly windthrown after the disease rots off roots just below the root collar, forming a "root ball." Early in the decay process, crescent-shaped or semicircular reddish brown staining of the wood may be observed. As decay progresses, the wood softens. The earlywood disintegrates more quickly than the latewood in each annual ring, resulting in a laminated ring rot where the annual rings of the wood separate. Small oval pits appear on both sides of the separated wood sheets. This disease forms 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 in an old center, regeneration. 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, but will not show crown symptoms until half to 75% of the roots are infected. Root rot centers may be many acres in size, and spread out at the rate of about 1-2 feet per year.

Fungus: Fruiting bodies (conks) are uncommon, but when found are located in protected areas such as on upturned roots and on the underside of decayed logs. Conks are flattened and range in color from buff to dark brown, with a white margin. The exposed surface is covered with many small pores. Reddish brown to brown whiskery mycelia (fungal fibers) may be observed between sheets of decayed wood, and white to purple-grey mycelial sheaths may be observed on outer bark surface of roots.

May be confused with: Armillaria root rot, Annosus root and butt rot, or animal damage.

Disease cycle: Laminated root rot is considered to be the most damaging root disease in the Pacific Northwest, as it kills the greatest concentrations of trees in the areas where it is present. There appears to be two distinct forms, one that causes a root disease in Douglas-fir, grand fir, and hemlock, and another form that causes a butt rot of western redcedar. The western redcedar form has only been identified on the east side of the Cascades, although western redcedar on the west side occasionally becomes infected with the Douglas-fir form, to which it is tolerant. The disease is spread by root contact between a healthy and infected individual. Mycelia of the fungus
do not grow through the soil, nor are windblown spores a major factor in disease spread. The fungus may remain viable in stumps for 50 years and thus infect regeneration, although it typically takes 10 to 15 years for root contact with the new trees to be established. The disease kills susceptible hosts by either predisposing them to windthrow by rotting the major roots, or by destroying their ability to take up water and nutrients. Saplings and small poles are usually killed quickly, while older trees may confine the fungus to a small number of roots or to the butt log and survive for many years. Trees of intermediate susceptibility are often infected but rarely killed,
while tolerant species are seldom infected and almost never killed.

Predisposing agents: Although all conifer species can be infected by laminated root rot, susceptibility varies. Old infected stumps from previously infected stands are a serious problem for regeneration of highly susceptible species. Certain silvicultural procedures such as commercial thinning or uneven-aged management may make infestations worse. The disease often occurs with other root diseases such as Armillaria root rot or Annosus root and butt rot. Laminated root rot is itself a predisposing agent for bark beetle attack.

Impact: On the west side of Oregon and Washington, laminated root rot causes annual losses of 32 million cubic feet of wood; east side damages could be equally high. The incidence of the disease has probably increased substantially with the suppression of fire and subsequent species shift to Douglas-fir/grand fir forests, as these species are highly susceptible to laminated root rot; ponderosa pine and western larch are more tolerant.

Management: Laminated root rot often occurs with other root diseases and predisposes trees to bark beetle attack. Always check apparent bark beetle kills for root disease signs as their presence may influence species regeneration and silvicultural methods. Control of laminated root rot is best attempted at the time of final harvest. Prior to harvest, the disease centers should be marked by examining outlying trees for characteristics of the disease (e.g. mycelia on the roots, reddish stain in the heartwood). 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, stumps should be examined outside the marked disease center for the characteristic red-brown semicircular staining; the disease center boundaries and buffer strips should be adjusted accordingly. It is important to check within a day or two of harvest as the stain fades quickly upon exposure to air. In Washington, the easiest, most cost-effective control is replacement of Douglas-fir/grand fir stands with other, less susceptible species. One 50-year rotation of tolerant, resistant or immune species such as ponderosa pine, western white pine, lodgepole pine, or western redcedar should result in the disease dying out on the site, provided that susceptible trees are not permitted to be reestablished. Another good alternative is a hardwood species such as alder, which is immune to the disease. A rotation of an intermediately susceptible species such as western larch may permit laminated root rot to remain on the site, though at a lower level than with highly susceptible species. Interplanting highly or intermediately susceptible trees with tolerant or rsistant trees will not help them survive; on the contrary, it is more probable that the tolerant trees will become infected. Susceptible species should not be planted within 100 feet of a disease
center. Another alternative is to treat inoculum by removing as many infected roots and stumps as possible. A bulldozer or excavator may be used to push out stumps or push over whole trees; on some sites explosives could be appropriate. Uprooted roots and stumps need not be burned as air drying kills the fungus.

In 10 to 15-year-old sapling stands with less than 25% of the area infected, thinning the diseased trees could be an effective measure. In young stands affected trees will appear scattered rather than in clear disease centers. All trees displaying symptoms, as well as all those adjacent to them (within two normal tree spacings, both high and intermediate in susceptibility), should be cut. This should break the disease pathway to healthy trees by killing the roots on which the disease spreads. It is important to cut the adjacent trees that do not show symptoms as the disease may be in an early stage and thus undetectable, or the tree may become infected later as its roots grow towards the disease center. When precommercially thinning near disease centers, always try to save tolerant or less susceptible species. Silvicultural treatments such as weed control or fertilization to maximize growth will neither help nor hinder the spread of the disease. Heavily infected sapling stands may be grown to harvestable small poles before losses become severe.

Where pole stands have numerous disease centers and more than 20% of the area is visibly affected, do not commercially thin; harvest at a younger rotation age. In pole stands which do not have numerous centers, or in which the centers are widely distributed, infected trees and nonsymptomatic adjacent trees should be harvested in a commercial thinning. The stand should be monitored for windthrown trees, which should be removed yearly to prevent their infestation by bark beetles.

Adapted from Hadfield, J.S. and D.W. Johnson, Laminated Root Rot. USDA Forest Service -Pacific Northwest Region, 1977, and from Hadfield, J.S. et al., Root Diseases in Oregon and Washington Conifers, USDA Forest Service-Pacific Northwest Region, 1986.

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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.