Pathogen
The native fungal pathogen Phaeolus occurs throughout North America and is commonly referred to as the velvet-top fungus or dyer’s fungus (Sinclair and Lyon 2005). The disease is generally referred to as red-brown butt rot or brown cubical butt rot (Boyce 1961; Sinclair and Lyon 2005). Previously, it was believed that P. schweinitzii was the sole species in North America but is now known to be limited to Europe and Asia (Cui et al. 2025). Presently, three species are recognized in North America, with P. hispidoides occurring across northern North America, P. tabulaeformis in the southeastern U.S., and P. occidentiamericanus in the southwestern U.S. (Cui et al. 2025).
Hosts
Phaeolus has a very broad host range among native and non-native conifers in North America and is widespread in both forests and managed landscapes (Overholts 1953; Boyce 1961; Hepting 1971; Sinclair and Lyon 2005). In the northeast, eastern white pine (Pinus strobus) is the primary host due to this tree’s natural abundance. The pathogen can also be regularly encountered on two- and three-needle pines (P. nigra, P. resinosa, P. rigida, and P. sylvestris), spruce (Picea; especially P. abies), and larch (Larix). Across southern New England, it is occasionally found on true firs (Abies) and only rarely on eastern hemlock (Tsuga canadensis).
Symptoms & Signs
Internal decay by Phaeolus primarily occurs in the roots and in the heartwood of the lower trunk (Sinclair and Lyon 2005). Because the outer vascular tissue remains healthy and intact, symptoms of infection are often nuanced or not present. In general terms, root and butt rot symptoms on conifers can manifest as gradual canopy dieback or thinning, off-color and stunted needles, premature needle shedding, excessive basal tapering (or flaring), resinosis from the lower trunk, and occasionally a stress cone crop (Sinclair and Lyon 2005). Typically, brown cubical root and butt rot goes undetected until trees suffer from dieback due to root loss, or lower trunk failure under loading from strong winds.
Annual fruiting bodies develop at the base of infected trees or from nearby lateral roots. They can appear from early August through September in southern New England. The mushrooms develop on a short, thick stalk, or form overlapping shelves when produced directly on the lower trunk. The upper surface of the cap (pileus) is yellowish-brown to reddish-brown and feels velvety when fresh. The underside (hymenium) is a similar color and has angular-shaped pores. Shortly after maturation, especially when exposed to near freezing temperatures, the mushrooms become dark reddish-brown, brittle, and can blend into the surrounding organic matter. While no longer viable, they often persist through the winter into the following spring. Once advanced decay has developed, fruiting bodies often develop at the base of infected trees each year.
Damage & Pattern of Decay
Phaeolus is a brown rot pathogen, meaning the cellulose and hemicellulose are preferentially targeted while the lignin remains in a modified form (Blanchette 1991). In the early stages of decay, infected wood tissue appears yellowish-brown, becoming reddish-brown as the decay advances (Sinclair and Lyon 2005). In the later stages of the decay process, infected wood is brown, very dry, and separates easily into cubical sections (Boyce 1961). Thin sheets of white mycelia develop in the fractures and cracks and can be used to help identify the fungus (Overholts 1953).
According to Boyce (1961; p 436), Phaeolus is responsible for “the most serious butt rot of conifers in the United States both as to the number of trees attacked and the volume of wood destroyed.” The degradation of cellulose and hemicellulose results in serious reductions in wood bending strength and advanced decay frequently leads to transverse fracturing (Sinclair and Lyon 2005). This can result in lower trunk breakage and uprooting under loading from strong winds. Decay columns are often concentrated on the lower portions of the trunk but at times can advance to exceptional heights (Boyce 1961).
Management
It’s believed that basal trunk wounds are a primary source of infection (Boyce 1961) and yet butt rot without associated root decay is rare (Sinclair and Lyon 2005).Phaeolus can likely colonize dead rootlets, injured larger roots, and various wounds or scars on the lower trunk (Sinclair and Lyon 2005). Given the tendency for trees in managed landscapes to suffer from abiotic wounding (Morgenroth et al. 2015), injured trees may be at higher risk for infection. It has been postulated that Phaeolus can colonize roots previously infected by Armillaria (Barrett 1970). Despite these various means of colonization, Phaeolus is frequently cited as a weak or opportunistic pathogen.
Active management of conifers infected by Phaeolus is challenging as there may be no symptoms of infection and trees can appear robust and healthy. If fruiting bodies are appearing annually, trees must be carefully monitored. Once Phaeolus is confirmed, understanding the severity of the decay can be performed using minimally invasive detection techniques (i.e. resistance drilling or sonic tomography). Yet, even with advanced detection methods, determining the extent of root damage is often impossible. Decay can develop in the heartwood and expands outward slowly over time. When the decay is confined to the center of the trunk and an intact shell of healthy wood surrounds the damage, infected trees may remain structurally stable. However, a thorough risk assessment should be performed for conifers suspected, or known, to harbor root and butt rot from Phaeolus.
Infected stumps and primary lateral roots should be removed from the site once the diseased tree is removed. Phaeolus can survive on stumps and dead roots for more than 15 years in some cases, allowing mushrooms to form and disperse spores locally (Barrett 1985). Avoid planting conifers like eastern white pine in shallow and poorly drained soils as these site conditions may predispose trees to infection (Sinclair and Lyon 2005)
References
Barrett DK. 1970. Armillaria mellea as a possible factor predisposing roots to infection by Polyporus schweinitzii. Transactions of the British mycological Society 55(3): 459–462.
Barrett DK. 1985. Basidiospores of Phaeolus schweinitzii: a source of soil infestation. European Journal of Forest Pathology 15(7): 417–425.
Blanchette RA. 1991. Delignification by wood-decay fungi. Annual Review of Phytopathology 29(1) 381–403. https://doi.org/10.1146/annurev.py.29.090191.002121
Boyce JS. 1961. Forest Pathology, 3rd edn. McGraw-Hill Book Company, Inc., New York, NY.
Cui YJ, Wang CG, Dai YC, Liu S, Ren YH, Schultes NP, Kaishian PO, Paine E, Yuan Y, Li DW, and Zhao H. 2025. Phylogeny, divergence times, and biogeography of the phytopathogenic fungal genus Phaeolus (Basidiomycota, Polyporales). Journal of Systematics and Evolution. https://doi.org/10.1111/jse.13187
Hepting GH. 1971. Diseases of Forest and Shade Trees of the United States. Washington, DC: USDA Agricultural Handbook No. 386.
Luley CJ. 2022. Phaeolus schweinitzii. Pp. 62-63 In Wood Decay Fungi Common to the Northeast & Central United States, 2nd Edition. Urban Forest Diagnostics LLC, Naples, NY.
Morgenroth J, Santos B, and Cadwallader B. 2015. Conflicts between landscape trees and lawn maintenance equipment – The first look at an urban epidemic. Urban Forestry & Urban Greening, 14(4): 1054–1058. https://doi.org/10.1016/j.ufug.2015.10.002
Overholts LO. 1953. The Polyporaceae of the United States, Alaska, and Canada. University of Michigan Press, Ann Arbor, MI.
Sinclair WA and Lyon HH. 2005. Diseases of Trees and Shrubs, 2nd edn. Cornell University Press, Ithaca, NY.