Schweinitzii butt rot
- French disease name: Carie brune cubique du pied
- Pathogen name: Phaeolus schweinitzii (Fr.) Pat. (Common names for the fungus: velvet-topped polypore, dyer’s polypore)
- Kingdom: Fungi
- Phylum: Basidiomycota
- Class: Agaricomycetes
- Order: Polyporales
- Family: Laetiporaceae
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Partial list of synonyms:
- Polyporus schweinitzii Fr.
General information and importance
Schweinitzii butt rot is the most serious disease of its type in old-growth North American coniferous forests. The causal agent, Phaeolus schweinitzii, is pathogenic to a wide range of hosts, both coniferous and broadleaved. It is the most common cause of brown cubical rot of Douglas-fir (Pseudotsuga menziesii).
Distribution and hosts
Phaeolus schweinitzii is found across Canada, reported from Alberta, British Columbia, Ontario, New Brunswick, Newfoundland and Labrador, Prince Edward Island, and Quebec. It has a worldwide distribution. Many tree species native to Canada in the following genera are hosts: true firs (Abies), larch (Larix), spruce (Picea), pine (Pinus), Douglas-fir (Pseudotsuga), yew (Taxus), cedar (Thuja), and hemlock (Tsuga). One broadleaved species, Garry oak (Quercus garryana), has also been reported as a host in Canada.
Phaeolus schweinitzii has been occasionally also reported on broadleaved hosts such as birch (Betula), plum and cherry (Prunus), and elm (Ulmus), and others in North America and elsewhere. Conifers have become infected by P. schweinitzii when planted on sites where formerly only hardwoods grew.
Tree parts affected
The roots, butt, and up to 3 metres of the lower trunk of the tree can be colonized.
Symptoms and signs
Evidence of Schweinitzii butt rot in conifer stands includes the presence of wind-thrown conifers with exposed brown cubical rot, in addition to the presence of fruiting bodies, either at the base of the tree, on the butt, or on the ground above the root system of the tree. If the tree is extensively diseased, decreased crown growth and branch dieback may be evident, especially if the tree is on a poor site.
Fruiting bodies of P. schweinitzii are produced annually, but because they are large and have a fairly tough consistency, they may persist for a year or two before they disintegrate. Common names for the fungus are “velvet-topped polypore”, which describes the upper surface of the fruiting body, or “dyer’s polypore”, because they can be used to dye fabrics a variety of hues depending on how they are prepared. Fruiting bodies are produced either on the ground, connected by a cord of mycelium to the host tree root system, or at the base of the tree or stump, and occasionally grow on the lower trunk of old dead trees. They are usually stalked, either solitary and funnel-shaped or in a cluster shaped like a rosette on the ground, or in layers of shelves when fruiting on the butt of the tree. They grow up to 25 centimetres in diameter, with central stalks up to 5 centimetres in diameter. Fruiting bodies growing from the ground often engulf nearby plant stems or other objects as they expand. The upper surface of the fruiting body is dark reddish-brown, bumpy, with concentric furrows and a coarsely velvety texture. Young fruiting bodies and the edges of older growing conks are usually greenish yellow, but some variants have bright orange edges. The lower surface of the fruiting body is greenish yellow at first, then darkens to greenish brown, then finally dark brown when fully mature. Pores on the undersurface are round or maze-like, with a density of 1 to 3 pores per millimetre. The hyphal system is monomitic, consisting of hyphae measuring 3 to 17 micrometres in diameter. The hymenium consists of a thin layer of cells covering the inner surface of the pores and has a distinctive feature of brown gloeoplerous hyphae (hyphae with oily contents). Basidia in the hymenium each bear four hyaline, smooth, ellipsoid to oval basidiospores measuring 6 to 8 micrometres × 4 to 5 micrometres. The mycelium of P. schweinitzii also forms thick-walled resistant spores called chlamydospores.
Infected wood is at first yellowed, then becomes brown and cracked into cubes as the fungus breaks down the cellulose, leaving behind the brittle dark lignin. Brown, thin resinous mycelial felts can be found in the cracks of the wood. Although most decay is within the roots and butt of the tree, decay can extend 3 metres up in the trunk. Advanced decay might be confused with brown crumbly rot or, in western redcedar (Thuja plicata), with decay caused by Oligoporus sericeomollis.
Disease cycle
All aspects of the disease cycle are not yet fully understood. Fruiting bodies are produced annually in late summer, or autumn in regions with dry summers. Basidiospores produced by the fruiting bodies are released from the pores and become airborne. If they land on soil, they can become established and grow mycelium in the soil in the absence of a host. Chlamydospores are also produced by mycelium in soil. The role of soil colonies in starting new infections has not been fully documented, but most initial infections are thought to be from basidiospores. As adjacent infected trees have been shown to host different clones of the fungus, root-to-root contact is not considered a mode of infection. Diseased trees frequently have scars from previous root or butt damage, which can serve as an infection court; in particular, old fire scars are very common on conifers with Schweinitzii butt rot. Logging scars are another locus for infection. Root tips can be infected and killed when they come into contact with the fungus, causing the remaining portions to become swollen and stubby, with resinous centres.
Damage
Trees of all ages may become infected by P. schweinitzii, but most commonly trees older than saplings are killed, and the highest levels of decay and damage are in trees older than 100 years. The reduction in size of root ends, and brown cubical decay in major structural roots makes infected trees susceptible to windthrow or breakage of the lower trunk. Infections predispose the trees to other more aggressive pathogens such as Armillaria. Large trees with advanced decay will have high levels of cull in the butt log.
Prevention and management
Forest management practices that reduce fire and mechanical damage to the tree base in mature conifer stands would reduce incidence of Schweinitzii butt rot.
Trees bearing fruiting bodies (or with fruiting bodies on the ground above their root systems) should be assessed for potential failure if they are close to buildings or are in recreational areas where they might pose a hazard to people.
Pest management strategies for a particular pest vary depending on several factors. These include:
- the population level of the pest (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
Fruiting body of Phaeolus schweinitzii on the stump of eastern hemlock.
André Carpentier
Fruiting body of Phaeolus schweinitzii on eastern hemlock.
André Carpentier
Advanced decay of Douglas-fir caused by Phaeolus schweinitzii showing brown cubical rot.
Duncan Morrison
Advanced decay of Douglas-fir caused by Phaeolus schweinitzii showing brown cubical rot.
Eric Allen
Advanced decay of Douglas-fir caused by Phaeolus schweinitzii showing brown cubical rot.
Eric Allen
Fruiting bodies of Phaeolus schweinitzii engulfing balsam fir seedlings.
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.
Ginns, J. 2017. Polypores of British Columbia. Province of British Columbia. Victoria, British Columbia. Technical Report 104.
Hagel, S.K.; Filip, G.M. 2010. Schweinitzii root and butt rot of western conifers. United States Department of Agriculture, Forest Service. Washington, D.C. Forest Insect and Disease Leaflet 177. 8p.
Hiratsuka, Y.; Chakravarty, P.; Bohning, R.A. 1996. A field guide to decay and stain of conifers in Alberta. Natural Resources Canada, Canadian Forest Service, Northern Forestry Centre. Edmonton, Alberta. Canada-Alberta Partnership Agreement in Forestry Report 142. 33 p.
Rippy, R.C.; Stewart, J.E.; Zambino, P.J.; Klopfenstein, N.B.; Tirocke, J.M.; Kim, M.S.; Thies, W.G. 2005. Root diseases in coniferous forests of the inland west: Potential implications of fuels treatments. United States Department of Agriculture, Forest Service, Rocky Mountain Research Station. Fort Collins, Colorado. General Technical Report RMRS–GTR–141. 21 p.
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.
Thomas, G.P.; Thomas, R.W. 1954. Studies in forest pathology. XIV. Decay of Douglas-fir in the coastal region of British Columbia. Canadian Journal of Botany 32(5): 630–653. https://doi.org/10.1139/b54-060
Whitney, R.D. 1995. Root-rotting fungi in white spruce, black spruce, and balsam fir in northern Ontario. Canadian Journal of Forest Research 25(8): 1209–1230. https://doi.org/10.1139/x95-134