Pathogen
The fungal pathogen Bondarzewia berkeleyi is native to eastern North America and is known as Berkeley’s polypore (Gilbertson and Ryvarden 1986). In older literature, the disease is referred to as string and ray rot (Long 1913; Boyce 1961). The genus Bondarzewia contains numerous species that occur throughout the world (Chen et al. 2016).
Hosts
Bondarzewia berkeleyi primarily occurs on oak (Quercus), but is also noted on chestnut (Castanea), maple (Acer), and cherry (Prunus) (Overholts 1953; Gilbertson and Ryvarden 1986). In southern New England, white oak (Q. alba), red oak (Q. rubra), and black oak (Q. velutina) are the most common hosts. Root and butt rot on conifers in western North America is caused by B. occidentalis (Chen et al. 2016) but was previously attributed to B. berkeleyi (Overholts 1953; Boyce 1961) and B. mesenterica (syn. B. montana) (Gilbertson and Ryvarden 1986).
Symptoms & Signs
Bondarzewia is a destructive and somewhat common decay pathogen of oaks in forests and managed landscapes (Luley 2022). Its occurrence and the resulting damage often go undetected, as many infected trees do not present noticeable symptoms. Because the decay occurs in the heartwood, excessive tapering at the base (or basal flaring) may be the only visible symptom. In some cases, additional symptoms on the lower trunk may include cracks, seams and sap flow. Infected trees often have healthy canopies as the sapwood remains healthy and intact (Long 1913).
Bondarzewia produces highly conspicuous, annual mushrooms consisting of one to five overlapping, cream- to tan-colored caps on a short central stem (Overholts 1953; Gilbertson and Ryvarden 1986). The pore layer is typically white, sometimes yellowish-brown in color, with angular pores, and typically does not bruise when pressed (Overholts 1953; Luley 2022). At times, the fruiting bodies can be several feet in width. However, on exposed sites with dry and compacted soils, the fruiting bodies may be distorted and difficult to identify. The mushrooms typically grow from infected roots a few feet away from the main trunk but can also grow directly from the main trunk at the soil line. In southern New England, mushrooms can appear anytime from mid-July through late September but tend to be most abundant in August. After maturation (e.g. 10–14 days), the mushrooms tend to darken and become dry and brittle. Most spores are dispersed locally and the risk of overland spread to other nearby oaks is likely low. Long (1913) believed that most infections initiated at the base of the trunk through various wounds.
Damage & Pattern of Decay
Bondarzewia causes a white stringy rot of the primary lateral roots and heartwood in the lower trunk, but typically does not extend upwards beyond 3–5 feet (Long 1913; Hepting 1971). Depending upon the source, the rate of decay is considered both slow and fast compared to other wood-rotting fungi. In the original description of the disease, Long (1913) described B. berkeleyi as progressing slowly within infected oaks, with significant decay occurring over many decades. Meanwhile, Hepting (1971) considered the fungus capable of a relatively fast rate of decay based on several studies from southern Appalachian oak forests.
Bondarzewia is a white rot fungus that exhibits a pattern of decay known as selective delignification. During this process, the compound middle lamella (the lignified matrix connecting cell walls along with the primary cell wall) is preferentially degraded before the cellulose-rich secondary cell walls are targeted (Blanchette 1991). When lignin is preferentially degraded, the wood tissue loses stiffness and compressive strength (Schwarze et al. 2000). During the early stages of decay by B. berkeleyi, the wood tissue may appear yellowish with wood fibers separating longitudinally (Long 1913). However, because the cellulose remains largely intact, infected wood experiences only minor reductions in bending strength. As the decay becomes more advanced and the hemicellulose and cellulose are degraded, the wood becomes reddish-brown and continues to separate into long, stringy fibers (Long 1913). There may be white, cottony sheets of fungal mycelia present. Finally, the decaying wood becomes more bleached, wet and spongy as it collapses to form a cavity within the heartwood.
One interesting feature is that the decay stays relatively confined to the primary lateral roots and lower trunk of the tree, rarely extending to great heights in the trunk (Long 1913; Luley 2022). As the decay expands outward from the heartwood, the base of the tree may dramatically swell to compensate. Ultimately, serious reductions in structural stability can result from advanced infections, making trees susceptible to uprooting or lower trunk failure under loading from strong winds.
Detection & Management
Bondarzewia is most often confirmed by the presence of its distinctive mushrooms. When they are absent, the most common symptom (excessive tapering of the lower trunk) is not diagnostic of the pathogen by itself. Therefore, carefully scout for the fruiting bodies during late summer and early autumn to confirm B. berkeleyi is present. Minimally invasive detection techniques, such as resistance drilling or sonic tomography, are often required to understand the extent of the damage. Yet, even with advanced detection methods, determining the severity of root damage is often impossible. Infected trees should receive a thorough risk assessment before any considerations on removal are made.
Maintaining high tree vigor is the best course of action when the fungus is known to be present. When internal decay is restricted to the heartwood, an intact shell of healthy wood can provide structural stability. Furthermore, when trees experience decay in the heartwood, they may endure if their annual increment growth can keep pace with the rate of internal decay. Thus, avoid drought stress by providing supplemental water during extended dry periods. Fertilization, maintaining optimal soil pH, and avoiding needless physical wounding from mowers and string trimmers will also promote vigor. Maintaining a large, mulched area around the tree will limit wounding to lateral roots, moderate soil temperatures, and reduce competition for moisture with turfgrasses.
Citations
Boyce, JS. 1961. Forest Pathology, 3rd edn. McGraw-Hill Book Company, Inc., New York, NY.
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
Chen J, Cui B, He S, Cooper JA, Barrett MD, Chen J, Song J, and Dai Y. 2016. Molecular phylogeny and global diversity of the remarkable genus Bondarzewia (Basidiomycota, Russulales). Mycologia 108(4): 697–708. https://doi.org/10.3852/14-216
Hepting GH. 1971. Diseases of Forest and Shade Trees of the United States. Washington, DC: USDA Agricultural Handbook No. 386.
Gilbertson RL and Ryvarden L. 1986. Bondarzewia. Pp. 171–174 In: North American Polypores. Fungiflora, Oslo, Norway.
Long WH. 1913. Three undescribed heart-rots of hardwoods trees, especially of oak. Journal of Agricultural Research 1(2): 109–130.
Luley CJ. 2022. Bondarzewia berkeleyi. Pp. 66–67 In: Wood Decay Fungi Common to the Northeast & Central United States, 2nd edn. Urban Forest Diagnostics LLC, Naples, NY.
Overholts LO. 1953. Polyporus berkeleyi. Pp. 238–240 In: The Polyporaceae of the United States, Alaska, and Canada. University of Michigan Press, Ann Arbor, MI.
Schwarze FWMR, Engels J, and Mattheck C. 2000. Fungal Strategies of Wood Decay in Trees. Springer, Berlin, Germany. https://doi.org/10.1007/978-3-642-57302-6