Pathogen
The fungus Rigidonotus glomeratus (formerly Inonotus glomeratus and Polyporus glomeratus) is responsible for canker-rot (Wu et al. 2022).
Hosts
Throughout its range across northern North America, R. glomeratus has been documented on maple (Acer), birch (Betula), chestnut (Castanea), beech (Fagus), poplar/cottonwood (Populus), oak (Quercus), and Willow (Salix) (Hepting 1971, Sinclair and Lyon 2005). In the northeastern U.S, it is most abundant on red maple (A. rubrum), sugar maple (A. saccharum), and American beech (F. grandifolia) (Boyce 1933, Overholts 1953).
Symptoms & Signs
Rigidonotus often infects branch stubs and pruning wounds on the main trunk of susceptible trees. The fungus then colonizes the heartwood, where it causes a serious delignifying white rot that can reduce the structural stability of the tree. Wood at advanced stages of decay can be soft and fibrous and often appears light brown to rusty brown in color. The brown coloration could potentially lead to confusion that a brown rot pathogen is present. Over time, the infection advances outward into the sapwood, resulting in canopy dieback.
Once advanced infections develop, the pathogen creates swollen, sterile conks that are easily missed without careful scouting. The sterile conks appear as rounded, black-colored, cracked to corky masses that are often surrounded by swollen and splitting bark tissue. The sterile conks do not disseminate spores and spore-bearing structures typically are not produced until after the tree is dead. The sterile conks rarely protrude more than a few inches from the trunk and may be mistaken for old branch stubs. There may be considerable cankering and wound wood around the site of the conk, which may not be present at all in some cases. According to Silverborg (1954), the presence of the sterile conk indicates a decay column (above and below the conk) of 5–6’ for beech, 5–7’ for sugar maple, and 6–8’ for red maple.
Management
On beech and maple with old pruning wounds, carefully scout nearby portions of the trunk for the presence of the sterile conks. They can be difficult to find when the foliage is present in the canopy. Tree climbers should familiarize themselves with the appearance of the conks so they can be readily identified if encountered. Little can be done to target the pathogen itself since the damage occurs in the heartwood where fungicides cannot penetrate. Pruning to reduce canopy sway can minimize bending stresses that could expose the structural weakness. Because the sterile conks do not produce and disseminate spores, they do not pose a risk to nearby maple and beech. It’s unclear how common Rigidonotus is on beech and maple in managed landscapes.
Citations
Boyce, JS. 1961. Forest Pathology, 3rd edn. McGraw-Hill Book Company, Inc., New York, NY.
Hepting GH. 1971. Diseases of Forest and Shade Trees of the United States. Washington, DC: USDA Agricultural Handbook No. 386.
Sinclair WA and Lyon HH. 2005. Diseases of Trees and Shrubs, 2nd edn. Cornell University Press, Ithaca, NY.
Silverborg, SB. 1954. Northern Hardwoods Cull Manual. State University of New York, Bulletin 31. 45 p.
Wu F, Zhou LW, Vlasák J, and Dai Y-C. 2022. Global diversity and systematics of Hymeanochaetaceae with poroid hymenophore. Fungal Diversity 113, 1–192. https://doi.org/10.1007/s13225-021-00496-4