White spongy trunk rot

Written by B.E. Callan Revision 2024

• French disease name: Carie blanche spongieuse du tronc
• Pathogen name: Fomes fomentarius (L.) Fr. (Common name for this fungus: tinder conk)
• Kingdom: Fungi
• Phylum: Basidiomycota
• Class: Agaricomycetes
• Order: Polyporales
• Family: Polyporaceae

General information and importance

Fomes fomentarius causes a white spongy wood rot of mature hardwood trees in boreal and temperate regions worldwide. It is most encountered on birch (Betula) and beech (Fagus) but has a broad host range outside these species. The presence of its characteristic pale woody conks indicates advanced decay and that there is little sound wood remaining in the host tree. “Tinder conk,” the common name of the pathogen, refers to the traditional use of the inner conk tissue for starting fires and transporting embers from one location to another. It has been used for this purpose since prehistoric times. The tissue under the hard exterior of the conk can also be peeled into strips, soaked and treated with various chemicals, then pounded into an absorbent flexible suede-like material called amadou. Amadou is used for drying fishing flies and for making hats and other items.

Distribution and hosts

Fomes fomentarius is widespread throughout the natural range of its hosts in all forested Canadian provinces and territories. It occurs on both live and dead hardwoods and has also been occasionally reported on conifers. In North America, it also occurs in the northern half of the United States, whereas in the southern half a closely related species, F. fasciatus predominates. Worldwide it has also been reported from Africa, Asia, and Europe.

Fomes fomentarius is particularly common in overmature birch and alder (Alnus) forests. The main hosts are white birch (B. papyrifera), yellow birch (B. alleghaniensis), water birch (B. occidentalis), and grey birch (B. populifolia). Other hardwood hosts in Canada include striped maple (Acer pensylvanicum), red maple (A. rubrum), sugar maple (A. saccharum), American beech (Fagus grandifolia), beech, ironwood (Ostrya virginiana), largetooth aspen (Populus grandidentata), trembling aspen (P. tremuloides), black cottonwood (P. trichocarpa), pin cherry (Prunus pensylvanica), Douglas-fir (Pseudotsuga), willow (Salix), oak (Quercus), eastern hemlock (Tsuga canadensis), and white elm (Ulmus americana).

Host parts affected

Heartwood and sapwood of the tree trunk and occasionally larger woody branches are susceptible to decay. Conks (fruiting bodies) develop on the trunk, often near old branch stubs and wounds, first establishing in sapwood, then colonizing the heartwood as the decay column in the trunk of the tree expands.

Symptoms and signs

Conks are perennial, woody, and found as a single cap or in a cluster on trunks of living and standing dead hardwood trees. Mature conks, which can continue to grow and produce annual pore layers for 30 years or more, are hoof-shaped, can reach 20 centimetres tall (parallel to the long axis of the trunk) × 15 centimetres wide, and project as much as 15 centimetres from their junction with the host bark. The upper surface of the conk (pileus) is covered with a smooth, non-cracked, matte, hard crust that is concentrically furrowed in various shades of gray, becoming brown in areas closest to the pore layer, which is the area of newest growth. The interior tissues of the conks are brown and react to potassium hydroxide (KOH; a drop of 5% KOH aqueous solution, a common mycological staining reagent), by first darkening, then fading to brown. The surface of the conk also shows a colour change to KOH, turning dark red. Conk tissues are trimitic, comprising three types of hyphae: (1) hyaline (colourless) thin-walled generative hyphae with clamp connections; (2) brown, thick-walled skeletal hyphae measuring 3 to 8 micrometres in diameter; and (3) hyaline to brown, highly branched and non-septate binding hyphae measuring 1.5 to 3.0 micrometres in diameter. Core tissue of the conks contain sclerids, which are contorted, brown, and very thick-walled cells measuring 30 to 70 micrometres × 10 to 30 micrometres.

The undersurface of the conk is brown and covered by round pores occurring at a density of 3 to 5 pores per millimetre. The pores are lined with the hymenium (spore-bearing tissue), which comprises club-shaped basidia interspersed with fusoid cystidioles (sterile cells). Each basidium bears four basidiospores. Basidiospores are hyaline, narrowly ellipsoid, smooth, thin-walled, 12 to 20 micrometres × 4 to 7 micrometres, and do not stain in either Melzer’s reagent or KOH.

Phellinus tremulae is known as the “false tinder conk.” It and other members of the P. igniarius species complex also produce hoof-shaped conks on hardwoods. They are macroscopically distinguished from Fomes fomentarius by their darker cracked exterior surfaces. Other than P. tremulae, other members of the P. igniarius complex do not produce sclerids.

Decay occurs in both the sapwood and heartwood. In the first stage of decay, brown discolouration is visible while the wood is still firm. Advanced decay is soft, spongy, and yellowish white, interspersed with thin black zone lines. As the decayed wood cracks, the resulting spaces are filled with yellowish white mycelium, which gives the wood a mottled appearance.

Disease cycle

Fomes fomentarius conks produce massive amounts of spores, with highest sporulation levels in the spring. A mature conk can produce up to 10¹¹ basidiospores per growing season, which en masse would be heavier than the conk itself. Spore discharge occurs from late spring to early summer. Spores are windborne and infect trees through broken branches (especially after storm events) and trunk wounds. Spores infect the sapwood, progress to the heartwood, and then expand vertically (both up and down) in the tree. It does not infect freshly wounded sapwood but establishes after pioneering bacteria and other fungi have colonized the wood. When the fungus decay column expands into the sapwood, it eventually reaches the cambium, and extensive cambial infections can cause crown death. The fungus continues to fruit and sporulate on dead and fallen trees. 

Damage

The presence of a single conk on a trunk indicates that there is little merchantable wood left in the tree. The fungus is opportunistic on declining overmature trees. It is the predominant wood decay fungus in yellow birch tree crowns in eastern Canada’s Maritimes region.

Prevention and management

Prevention of wounding on host trees during the growing season while the conks are actively releasing spores can decrease the risk of spread of the fungus. High-value trees in urban areas should be assessed for stem breakage risk or removed if conks are present on them. In managed hardwood forests, selectively harvesting overmature trees would reduce the potential for future establishment of decay columns in younger trees.

Pest management strategies for a particular pest vary depending on several factors. These include:

• the population level of the pest (i.e., how numerous the pest is on the affected host[s]);
• the expected damage or other negative consequences of the pest’s activity and population level (either to the host, property, or the environment);
• an understanding of the pest’s life cycle, its various life stages, and the various natural or abiotic agents that affect population levels;
• how many individual host specimens are affected (an individual tree, small groups of trees, plantations, forests);
• the value of the host(s) versus the costs of pest management approaches; and
• consideration of the various silvicultural, mechanical, chemical, biological, and natural control approaches available and their various advantages and disadvantages.

Decisions about pest management strategies require information about each of these factors for informed decision-making. These various factors should then be weighed carefully in terms of costs and benefits before action is taken against any particular pest.

Selected references

Allen, E.A.; Morrison, D.J.; Wallis, G.W. 1996. Common tree diseases of British Columbia. Natural Resources Canada, Canadian Forest Service, Pacific Forestry Centre. Victoria, British Columbia. 178 p.

Gilbertson, R.L.; Ryvarden, L. 1987. North American polypores Vol. 1. Abortiporus – Lindtneria. Fungiflora A/S. Oslo, Norway. 1–433 p.

Ginns, J. 2017. Polypores of British Columbia (Fungi: Basidiomycota). Province of British Columbia, Victoria, British Columbia. Technical Report 104. 260 p. https://www.for.gov.bc.ca/hfd/pubs/Docs/Tr/TR104.pdf  [Accessed August 2024]

Hillborn, M.T. 1942. The biology of Fomes fomentarius. Bulletin of the Maine Agricultural Experimental Station No. 409: 161–214.

Meyer, H. 1936. Spore formation and discharge in Fomes fomentarius. Phytopathology 26: 1155–1156.

McCormick, M.A.; Cubeta, M.A.; Grand, L.F. 2013. Geography and hosts of the wood decay fungi Fomes fasciatus and Fomes fomentarius in the United States. North American Fungi 8(2): 1–53.

Pegler, D.N. 2001. Useful fungi of the world: amadou and chaga. Mycologist 15(4): 153–154. https://doi.org/10.1016/S0269-915X(01)80004-5

Schwarze, F. 1994. Wood rotting fungi: Fomes fomentarius (L.:Fr.) Fr.: hoof or tinder fungus. Mycologist 8(1): 32–34. https://doi.org/10.1016/S0269-915X(09)80679-4

Sinclair, W.A.; Lyon, H.H. 2005. Diseases of trees and shrubs. Second edition. Comstock Publishing Associates, Cornell University Press. Ithaca, New York. 660 p.

Stillwell, M.A. 1954. Progress of decay in decadent yellow birch trees. The Forestry Chronicle 30(3): 292–298. https://doi.org/10.5558/tfc30292-3

Cite this fact sheet

Photos

Fruiting bodies of the fungus Fomes fomentarius on American beech.

André Carpentier

Fruiting bodies of the fungus Fomes fomentarius on white birch.

Eric Allen

A cross-section of the fruiting body of Fomes fomentarius and the stem of white birch with advanced decay.

Eric Allen