Brown cubical rot of birch
- French disease name: Carie brune cubique du bouleau
- Pathogen name: Fomitopsis betulina (Bull.) B.K. Cui, M.L. Han & Y.C. Dai (Common name for the fungus: razor strop fungus)
- Kingdom: Fungi
- Phylum: Basidiomycota
- Class: Agaricomycetes
- Order: Polyporales
- Family: Fomitopsidaceae
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Partial list of synonyms:
- Fomes betulinus (Bull.) Gillot & Lucand
- Piptoporus betulinus (Bull.) P. Karst.
- Polyporus betulinus (Bull.) Fr.
General information and importance
Fomitopsis betulina is a distinctive large, annual, smooth-topped, tan-coloured polypore found exclusively on birch (Betula) throughout the Northern Hemisphere. It is found wherever birch species occur in Canada and causes a brown cubical rot of the sapwood, especially on trees killed by fire or armillaria root disease. It is one of the few brown rotting fungi that exclusively attacks hardwoods in nature. A piece of F. betulina was found on “Ӧtzi the Iceman,” the mummified body of a man who died 5,300 years ago and was discovered in an Italian glacier in 1991. The fungus is attributed with numerous traditional medicinal properties and is the subject of modern studies for potential pharmacological use. The fruiting body is often called the “razor strop fungus” as the corky interior can be used to sharpen knife blades. It has also been used as a traditional source of tinder to start fires.
Distribution and hosts
Fomitopsis betulina is circumboreal in its distribution worldwide. In Canada, its distribution roughly follows that of white birch (B. papyrifera), with confirmed reports from all provinces and territories. Fomitopsis betulina is exclusive to birch. It fruits on mature, usually dead standing or fallen trees. In Canada, its main hosts are white birch in western Canada and yellow birch (B. alleghaniensis) in eastern Canada, but it also occurs on water birch (B. occidentalis) and gray birch (B. populifolia).
Tree parts affected
Standing trunks and fallen logs of dead birch trees.
Symptoms and signs
The large distinctive annual conks are kidney-shaped, pale brown to gray, and attached to the trunk by a short stout stalk. Conks grow up to 25 centimetres in width but are usually approximately 15 centimetres wide, and project shelf-like from their attachment point (3–15 centimetres wide), with a thickness of 1 to 5 centimetres (occasionally up to 10 centimetres). The upper conk surface is tough and leathery, gray to pale brown, with its smooth surface sometimes cracking to give a slightly scaly appearance. Underneath, the margin of the pore layer is bordered by a raised, curb-like edge. The pore layer is white to cream, 2 to 10 millimetres deep, and easily stripped from the context (inner conk tissue) when fresh but tending to split when dried to expose individual tubes. The context is white, tough, and corky when dried. Its pores are round to angular, 3 to 5 per millimetre. The hymenium lining the pore interior consists of club-shaped hyaline (colourless) basidia measuring 10 to 12 micrometres × 5 to 6 micrometres each bearing four basidiospores. Basidiospores are hyaline and allantoid (sausage-shaped), measuring 5 to 6 micrometres × 1.5 to 2.0 micrometres.
The conks comprise two types of hyphae (dimitic): thin-walled hyaline generative hyphae (2.5 to 4.0 micrometres in diameter) with clamp connections and thick-walled hyaline branched skeletal hyphae (2.5 to 5.0 micrometres in diameter).
The fungus causes a brown rot, preferentially degrading cellulose and leaving the brittle brown lignin components behind. Decayed wood is yellowish-brown and usually cracked and has an odour of apples, with thin white sheets of fungus mycelium in the cracks. The wood is light in weight and powders easily under pressure. Individual decay columns are separated by narrow zones of relatively undecayed wood. The decay rate is rapid, and the presence of fruiting bodies on dead trees indicates that the brown cubical rot is extensive in the trunk.
Disease cycle
The fruiting body develops, matures, releases its spores, and dies within a single growing season. In laboratory conditions on artificial media, the completion of the life cycle from spore to fertile pore layer takes three months. In birch wood artificially inoculated by F. betulina, a 30% to 70% reduction of wood density can be measured after four months. Growth from inoculation points was up to 20 centimetres after one year.
Basidiospores released from the pores on the undersurface of the conks are lifted and dispersed by wind currents. These basidiospores may infect new hosts if they land on broken bark caused by fire damage or damage by other means. Trees killed by armillaria root disease are particularly susceptible to colonization. Beetles and other insects feed extensively on conks the year after they die causing them to disintegrate.
Damage
Extensive decay can lead to stem failure of trees exposed to high winds or snowstorms. The presence of fruiting bodies on dead trees indicates that the brown cubical rot is extensive in the trunk, and that little sound wood remains.
Prevention and management
Dead trees with fruiting bodies are at high risk for failure and should be removed in urban settings or around high-use recreational areas. Brown cubical rot of birch is also beneficial as part of the natural recycling of woody debris from decadent stands of overmature trees. Cavity nesting animals often make use of the softened wood.
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.
Photos
Fresh fruiting body of the fungus Fomitopsis betulina on a dead standing gray birch stem.
André Carpentier
A relatively fresh (upper) and an older (lower left) fruiting body of the fungus Fomitopsis betulina on a dead standing birch stem.
Older fruiting bodies of Fomitopsis betulina on a fallen birch stem.
Selected references
Adams, T. 1982. Piptoporus betulinus: Some aspects of population biology. Ph.D. Thesis, University of Exeter. 246 p. https://www.researchgate.net/profile/Tim-Adams-5/publication/35964342_Piptoporus_betulinus_Some_aspects_of_population_biology/links/569f2b7c08ae2c638eb5aefd/Piptoporus-betulinus-Some-aspects-of-population-biology.pdf [Accessed August 2024]
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. 2: Megasporoporia – Wrightoporia. Fungiflora, Oslo, Norway. pp. 434–885.
Ginns, J. 2017. Polypores of British Columbia. Province of British Columbia. Victoria, British Columbia. Technical Report 104. https://www.for.gov.bc.ca/hfd/pubs/Docs/Tr/TR104.pdf [Accessed April 2024]
Macdonald, J.A. 1937. A study of Polyporus betulinus (Bull.) Fries. Annals of Applied Biology 24(2): 298–310. https://doi.org/10.1111/j.1744-7348.1937.tb05034.x
Pleszczyńska, M.; Lemieszek, M.K.; Siwulski, M.; Wiater, A.; Rzeski, W.; Szczodrak, J. 2017. Fomitopsis betulina (formerly Piptoporus betulinus): the Iceman’s polypore fungus with modern biotechnological potential. World Journal of Microbiology and Biotechnology 33: 83. https://doi.org/10.1007/s11274-017-2247-0
Scharpf, R.F. 1993. Diseases of Pacific coast conifers. United States Department of Agriculture, Forest Service. Washington, D.C. Agriculture Handbook 521 (revised). 199 p. https://archive.org/details/diseasesofpacifi521scha/page/n1/mode/2up [Accessed August 2024]
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.