Forest & Grassland Health

Stem Decays of Conifers

Caused by several fungi

Host(s) in Alaska:

All conifers


most decay heartwood, some occupy sapwood & heartwood






Click on the image for a larger version.

Conks of common stem decay fungi of Alaskan conifers.



Conks of several common stem decay fungi of Alaskan conifers. Also see this guide for more pictures and descriptions of common stem, root, and butt rot fungi of Alaska's trees, adapted from our Pocket Guide for the Identification of Common Forest Diseases and Insects in Alaska (Winton et al. 2017).

Click here to view stem decay fungi ID cards.

Current Status & Distribution in Alaska (2020 Update)

Stem decays occur on conifer hosts throughout the state, but have been studied in greatest depth in Southeast Alaska. The incidence of stem decays changes little over time without active management. In mature forests of Southeast Alaska, conifer stem decays cause enormous wood volume loss. Approximately one-third of the old-growth timber volume in Southeast Alaska is defective, largely due to stem decay. There is very little decay in young-growth stands unless there is prevalent wounding. Stem decays are key disturbance agents in the coastal rainforest, because they predispose large old trees to bole breakage and windthrow. Stem decays create canopy gaps, influence stand structure and succession, perform essential nutrient-cycling functions, increase biodiversity, and enhance wildlife habitat. Trees with stem decay can be hazardous in managed recreation areas. Brown rots are the most significant source of cull for Sitka spruce, while white rots are most significant for western hemlock and western redcedar. Western redcedar is the most defective species, followed by western hemlock and Sitka spruce.

A variety of different fungi cause stem decay in Alaskan conifers; this table shows the most common conifer stem decays in Alaska with information about the type of decay they cause, their tree hosts, modes of infection and known distributions in Alaska. For some conifer stem decay fungi, we have enough georeferenced ground observations to provide maps (see Distribution Maps). In 2015, the paint fungus (Echinodontium tinctorium), thought to be absent in Southeast Alaska south of Skagway, was found to be abundant on western and mountain hemlock in one stand on Mitkof Island south of Petersburg.

  • Brown Crumbly Rot, Fomitopsis pinicola
    Over 20 observations of Fomitopsis pinicola (red belt conk) were recorded in 2020 by FHP staff, with an additional 30 research grade observations from citizen scientists reported in iNaturalist. Many of our observations were associated with the substantial spruce beetle activity in the Matanuska-Susitna Valley on white spruce. In this area, many of the spruce beetle-killed trees snapped off in the lower bole. Almost all of them had red belt conks and brown, crumbly, decayed wood with mycelial mats in the cracks. It is assumed that the trees had been infected long before they snapped because of the extensive advanced decay. Two popular Southcentral Alaska state campgrounds were closed in 2019 (Byers Lake Campground and South Rolly Lake Campground in the Nancy Lakes State Recreation Area) because of hazard trees created by spruce mortality, but reopened in 2020. Fomitopsis pinicola is presumed to occur throughout the range of its hosts and has been recorded on all spruce and hemlock species in Alaska (Map 15 Fomitopsis pinicola).Two recent publications indicate that Fomitopsis pinicola is not a single species, but rather a cryptic species complex (Haight et al. 2016, Mycologia, 108(5), 2016, pp. 925–938; Haight et al. 2019, Mycologia, 111 (2), pp. 339-357). Phylogenetic analyses show that two species are likely present in Alaska: F. ochracea and F. mounceae. They co-occur in coastal Alaska, but only F. mounceae is present in Interior and Western Alaska. The red band that gives the conk its common name, red belt, is thought to be more characteristic of F. mounceae. Consequently, iNaturalist discontinued Fomitopsis pinicola as an accepted taxon, which has created some obstacles in interpreting citizen scientist reports that base identification on location or presence of the red belt. We will continue to call this fungus F. pinicola or F. pinicola sp. complex until there is broader adoption of the newly suggested scientific names.
  • Red Ring Rot, Porodaedalea pini
    Porodaedalea pini was recorded on white, black, and Sitka spruce at three locations in Southcentral and Interior Alaska in 2020. Near Juneau, eight occurrences were found on Sitka spruce and western and mountain hemlocks. Seven research grade observations were recorded in iNaturalist. An impressive patch of more than 30 affected mountain hemlocks was found on the slope of Mt. McGinnis above the Mendenhall Glacier in Juneau. Fruiting bodies often occur near branch stubs on live trees and are an indicator of heart rot. Extensive internal decay is often indicated by multiple fruiting bodies along the length of the bole. Although primarily considered a heart rot, P. pini can progress into sapwood and kill trees.
  • Brown Cubical Butt Rot, Phaeolus schweinitzii
    In 2020, Phaeolus schweinitzii was recorded at two locations in Southcentral Alaska on Sitka and white spruce, as well as two adjacent locations on Sitka spruce near Juneau. Ten research grade observations were contributed through iNaturalist. This fungus is particularly common on Sitka spruce in Southeast Alaska. The fruiting bodies are most noticeable in fall, emerging from decayed wood of broken tree boles or around the bases of the tree, connected to tree roots below ground. Damage can be most severe in areas with compacted or disturbed soils; for this reason, this fungus increases hazard tree issues at recreation sites, where infrastructure development or aggressive public use may inadvertently compromise tree roots and encourage infection. The brown cubical rot symptom of P. schweinitzii may easily be mistaken for that caused by the much more common Fomitopsis pinicola.

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Symptoms, Biology & Impacts

Stem decays rot or deteriorate wood, primarily in tree trunks, rather than roots and butts. They can be identified based on the presence and characteristics of conks, mushrooms, or other fungal structures on tree boles, when present. The characteristics of decayed wood and species of host tree can also be helpful for identification. Wildlife holes, cavities, and hollows indicate the presence of stem decay on live trees, even when conks and mushrooms are absent. Heart rot develops primarily in the heartwood (inner wood) of living trees, whereas sap rot develops in the sapwood (outer wood beneath bark) and is usually extensive only in dead trees. Bole wounds and cracks provide entry points for many stem decay fungi, although some decays enter through natural openings like branch stubs.

  • Brown rots are particularly detrimental to tree strength. They degrade cellulose fibers leaving behind brownish lignin, which dries in brittle cubes.
  • White rots decompose all wood components (cellulose and lignin); wood remains fibrous until very late stages of decay. The color and texture of white rots is dependent upon the causal fungi.
Brown rot on a snapped spruce bole. White rot of yellow-cedar.

Examples of cubical brown rot of a snapped Sitka spruce snag (left) and fibrous, stringy white rot on a recently-cut yellow-cedar stump (right).

By predisposing large old trees to bole breakage and windthrow, stem decays are key disturbance agents. Individual tree mortality, much of it caused by heart rot fungi, creates small-scale canopy gaps and appears to be the leading form of disturbance in the coastal rainforest (Hennon 1995), where fire and other large-scale disturbances are uncommon. All major tree species in Southeast Alaska have been found killed in this manner. Stem decays influence stand structure and succession, perform essential nutrient cycling functions, increase biodiversity, and enhance wildlife habitat. Heart rot has an obvious and essential role in wood decomposition and has been demonstrated to be a site of nitrogen fixation by other microorganisms. Cavities created by stem decay fungi in standing trees provide crucial habitat for many species (bears, voles, squirrels and birds). Stem decays reduce merchantable timber volume from mature harvest units (especially old-growth) and can be hazardous in managed recreation areas.

Many stem decay fungi cause heart rot of living trees, others decay the wood of dead trees, and some grow on dead tissue of both live and dead trees. Most of these decays do not actually interfere with the normal growth and physiological processes of live trees since the vascular system is unaffected. However, some decay pathogens, such as Phellinus hartigii and P. pini may attack the sapwood and cambium of live trees after existing as a heart rot fungus. Many of the fungi that are normally found on dead trees (e.g., Fomitopsis pinicola) can grow on large stem wounds, broken tops and dead tissue of live trees. Root and butt rot fungi can also cause stem decay in the lower bole (e.g., Phaeolus schweinitzii).

Wounds on live trees caused by logging activities are potential sites of infection for decay fungi to cause appreciable timber losses (Wright and Isaac 1956). Generally, larger, deeper wounds and larger diameter breaks in tops result in a faster rate of decay (Hennon 1990). Without logging injury, heart rot in young forests are typically at very low levels until stand age 100 to 150 years. Eventually, heart rot consumes as much or more wood volume annually than is produced by the live trees. There are methods that could be used to promote earlier development of stem decays for wildlife habitat in young-growth stands with non-timber objectives (Filip et al. 2011, Hennon and Mulvey 2014), such as intentional bole wounding and top breakage during stand entries. In some instances, bole breakage can be encouraged to occur in a specific direction (e.g., across steams for coarse wood debris input) by causing wounds to one side of the bole (e.g., the side that faces the stream).

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Historic Activity

An important cull study conducted by James Kimmey in Southeast Alaska in the 1950s found that brown rots were the most significant source of cull for Sitka spruce, while white rots were most significant for western redcedar (especially Obba/Ceriporiopsis rivulosa and Phellinus weirii) and western hemlock. Farr et al. (1976) found similar high rates of decay in old-growth forests as Kimmey (1956). These and other studies have shown that stem decay incidence and volume increase with tree size. The amount of defect also depends on tree species: for any given size or age class, redcedar was the most defective species, followed by western hemlock and Sitka spruce. Although redcedar wood products are known for decay resistance, it seems that a few species of decay fungi are specialized to overcome the decay resistance of live redcedar but do not affect wood in service.

Contribution of fungi that cause brown rot and white rot in Sitka spruce. Contribution of fungi that cause brown rot and white rot in western hemlock.

Contribution of fungi that cause brown rot (tan wedges) and white rot (light grey wedges) in living Sitka spruce (left) and western hemlock (right) in the forests of southeast Alaska. Adapted from Kimmey (1956) for Hennon and Mulvey (2014). Click on images for larger versions.

Decay incidence by tree age for Sitka spruce and western hemlock. Volume cull by tree age for Sitka spruce and western hemlock.

Influence of tree age on percentage of wood volume that is cull (left) and incidence of decay in trees (right). Adapted using data from Table 10 from Kimmey (1956) of mean gross volume cull values for dissected trees grouped by 50-year age intervals. Curves were fit with polynomial equations. Also used in Hennon and Mulvey (2014). Click on images for larger versions.

Survey Method 

Our knowledge of stem decay impacts on timber volume loss primarily come from two cull studies in Southeast Alaska (Kimmey 1956 and Far et al. 1976). A study by Hennon and McClellan (2003) evaluated modes of tree mortality (e.g., died standing, broken bole, uprooting) in old-growth forests. Permanent monitoring plots throughout Southcentral and Interior Alaska (evaluated 2013-2016) are helping to build information about the distribution and relative importance of stem decays on various tree hosts in other regions and forest types in Alaska.

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Distribution Maps 

Click on the image for a larger version.

Recent georeferenced observations of Echinodontium tinctorium in Alaska. Detection locations of Latiiporus sulphureus in Alaska as of 2019


Fomitopsis pinicola observations in Alaska as of 2020. Observations of Phaeolus schweinitzii in Alaska as of 2020.
Observations of Porodaedalea pini in Alaska as of 2020.

Georeferenced observations of stem decay fungi Echinodontium tinctorium, Laetiporus sulphureus, Fomitopsis pinicolaPhaeolus schweinitzii, and Porodaedalea pini in Alaska with the range of their hosts. Non-georeferenced but known occurences of these fungi are excluded, but are described in this table. Modeled tree host distributions were developed by the Forest Health Technology Enterprise Team in 2011 (240m-resolution, presence based on dominant tree species by tree diameter).

Links to Resources & Publications

Farr, W. A.; LaBau, V. J.; Larent, T.L. 1976. Estimation of decay in old-growth western hemlock and Sitka spruce in southeast Alaska. Research Paper PNW-204. Portland, OR: U.S. Department of Agriculture, Forest Service. 24 p.

Filip, G.; Chadwick, K.; Zambino, P.; and others. 2011. Seven- to 12-year effects of artificially inoculating living conifers to promote stem decay and subsequent wildlife use in Oregon and Washington forests. Portland, OR: USDA Forest Service, Forest Health Protection.

Hennon, P. E. 1990. Wounding on residual Sitka spruce and western hemlock remaining after thinning on Prince of Wales Island, Alaska. USDA Forest Service, State and Private Forestry, Juneau, AK. Forest Pest Management Report R10 90 2. 9p.

Hennon, P. E. 1995. Are heart rot fungi major factors of disturbance in gap-dynamic forests? Northwest Science. 69: 284-293. Available here.

Hennon, P.E.; McClellan, M. H. 2003. Tree mortality and forest structure in temperate rainforests of southeast Alaska. Canadian Journal of Forest Research 33: 1621-1634.

Hennon, P. E.; Mulvey, R. L. 2014. Managing heart rot in live trees for wildlife habitat in young-growth forests of coastal Alaska. Gen. Tech. Rep. PNW-GTR-890. Portland, OR: U.S. Department of Agriculture, Forest Service, Pacific Northwest Research Station. 23 p. Available here.

Kimmey, J. W. 1956. Cull factors for Sitka spruce, western hemlock, and redcedar in southeast Alaska. USDA Forest Service. Alaska Forest Research Center, Juneau, Alaska. Station Paper No. 6. 31p.

Kimmey, J. W. 1964. Heart Rots of Western Hemlock. USDA Forest Pest Leaflet 90. Available here.

Wright, E.; Isaac, L. A. 1956. Decay following logging injury to western hemlock, Sitka spruce, and true firs. USDA Tech. Bull. No. 1148. 34p.


Content prepared by Robin Mulvey, Forest Pathologist, Forest Health Protection,

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