Photo 1. Canker rots typically decay the heartwood and sapwood before they kill the bark and cambium and cause cankers. Note the depressed face of the stem, and that the decay tree has reacted by forming wound wood on the edges of the canker (arrow). Photo courtesy of Steve Robinson.

Canker rots? Most arborists don’t think of wood-decay fungi as a cause of cankers – defined simply as pathogens that kill the bark, cambium and outer xylem – of urban trees. In the case of canker rots, the fungi both decay wood and cause cankers. (Photo 1)

Photo 3. Canker rots may be overlooked as initial symptoms. Seams or bark cracking can be overlooked or not ascribed to a wood-decay fungus causing cankers. Globofomes graveolens, also commonly known as sweet knot, on red oak (shown here), is common on oaks and has been associated with trunk failures. All photos courtesy of the author, unless otherwise noted.

Canker rots are more common than is widely known and may be hiding in plain sight! (Photo 2) One reason is that several well-known decay fungi can cause canker rots but are usually not recognized for this type of attack. Canker-rot symptoms also may be easily overlooked, especially in the early stages of canker development. (Photo 3) Initial symptoms are subtle and could be attributed to other issues or diseases. However, when canker rots occur, they may require action to inspect and possibly remove infected trees because of the unique way the decay develops and impacts tree stability. (Photo 4)

Disease cycle
Canker-rot fungi can affect both larger tree branches and trunks. Initial infection occurs in wounds, small dead branches or sometimes by infecting cankers caused by other fungi. Spread from tree to tree is mostly by airborne spores released from conks or brackets. (Photo 5) Fungal fruiting typically appears in the later stages of disease, usually on severely decayed trees with cankers. Several canker-rot fungi produce sterile conks on living trees. Sterile conks produce no spores, and brackets with spores may not appear for many years after the trees are removed.

Modes of action
There are a number of ways that canker-rot fungi attack urban trees.

Cankers developing after extensive trunk decay might be considered the “classic” way canker rots develop. After initial infection, heartwood or inner sapwood is decayed before any cankers develop. The decay initially progresses inward from infection sites. Most canker rots cause extensive internal decay before external symptoms are evident. (Photos 1 and 4)

In the later stages of disease progression, these fungi progress outward toward the vascular cambium, where elongated, depressed cankers develop on the main trunk. Initial canker symptoms may be only a slightly depressed area on the trunk, or they may appear only as a crack or seam. (Photos 3 and 5)

More obvious perennial cankers with wound-wood margins are common as the tree attempts to wall off or respond to infection of the cambium. (Photo 6) A unique feature of cankers from this attack is that killing of the bark and cambium may stimulate the formation of Wall 4. Wall 4, or the barrier zone, is the strongest defense in a tree’s natural decay/wound response system. This is likely the only time that barrier-zone formation occurs outside of external wounding.

Fruiting often follows the appearance of cankers, as is common for Inonotus hispidus (Photo 5), I. andersonii (Photo 7) and Globifomes graveolens (Photo 3), all of which are common on oak species. By the time cankers and fruiting occur, internal decay is usually extensive.

Photo 2. Some canker rots may be hiding in plain sight, as a number of common, well-known decay fungi such as Fulvifomes (Phellinus) robiniae on black locust can cause cankers. Note the depressed area with wound wood below the perennial conks.

Cankers uncommon
Several well-known decay fungi also may follow this classic canker-rot pattern but are not commonly recognized as causing canker rots. For these pathogens, the cankering is more subtle or develops only as depressed areas in the trunk, often without formation of obvious perennial cankers. Examples include the well-known Fulvifomes (Phellinus) robiniae on black locust (Photo 2) and Laetiporus sulphureus, aka sulfur shelf or chicken of the woods, common on oaks and black cherry. These fungi can cause cankers on trunks or large-diameter scaffold branches after decay of heartwood or sapwood.

Sterile conks and cankers
Several canker rots produce sterile conks in or around cankers. Inonotus glomeratus and I. obliquus – aka the familiar Chaga – are the most well known on sugar maple and birch, respectively. (Photo 8) The sterile conks are intertwined with host tissues and contribute to the development of the perennial cankers by wedging in and attacking the edges of wound wood and barrier zones, allowing expansion of perennial cankers. Trees with sterile conks typically have extensive internal decay columns. Both these pathogens only form fertile, spore-
producing brackets after trees die, sometimes as long as seven years later on downed logs!

Cankers following infection of sapwood
Although not usually categorized with canker rots, several fungi cause cankers in a very different manner. These fungi first decay sapwood shortly after infecting trunks or branches through larger wounds. (Photo 9) Once established in sapwood, they attack bark and cambium and enlarge the initial wound or infection site. Decay progression is typically outside in, meaning outer sapwood is decayed first and the decay progresses inward into dead or altered sapwood.

In direct contrast to the “classic” canker rots, these fungi often fruit very early in the stages of decay development. They are known to be indicators of pending failure, especially on smaller-diameter branches. (Photo 10) On trunks, their progression is usually slower because of heartwood and larger volumes of trunk wood. On small-diameter trees, their appearance usually indicates a stressed tree that is unlikely to survive to maturity. Schizophyllum commune, Trametes versicolor (turkey tail) and Cerrena unicolor (Photo 10), among other species, are examples of this type of attack on bark and cambium early in the stages of decay. 

Photo 4. Trees with canker rots may require close inspection and removal. Note the extensive decay in this red oak infected with Inonotus hispidus. Photo courtesy of Margery Daughtrey, Cornell University Extension.

Photo 5. Canker rots are primarily spread by airborne spores released from fruiting structures associated with cankers. Wounds and dead branches are common sites of infections. Bracket of Inonotus hispidus, a common cause of canker rots on oak, prior to removal of the tree (see Photo 4). Photo courtesy of Margery Daughtrey, Cornell University Extension.

Conclusion
In most cases where canker rots appear, removal is likely a strong recommendation, given that cankers and fruiting develop only after heartwood or inner sapwood is extensively decayed. (Photos 4 and 6) There may be more latitude for trees without cankers with some fungi, but these should be inspected closely with sounding, probing or advanced testing methods.

Trees with sapwood decay/canker fungi are suspect, as these fungi typically are found on trees stressed or damaged by other agents. In the case of branch infections, the branches should be removed, as decay in outer wood fibers can quickly and dramatically impact stability. (Photo 11) The decay can progress rapidly, within a growing season, making any branch with large open wounds, even without fruiting bodies, unreliable for climbing or tying into. Although some of these fungi are common as saprophytes on dead trees and branches, they also are common on living branches and trees. (Photo 9)

Christopher J. Luley, Ph.D., is a consultant and tree pathologist in Naples, New York. He published an updated “Wood Decay Fungi Common to Urban Living Trees” in 2022. The field guide can be purchased at the International Society of Arboriculture’s bookstore at isa-arbor.com, or as an online read-only version at treerot.com.

Photo 6. Canker rots typically cause extensive internal decay before they move outward into sapwood and eventually attack the bark and cambium. Note that the sapwood reacts by forming repeated zones of discolored tissue (arrows) that have high levels of oxidized phenolic compounds known as reaction zones. Photo courtesy of Steve Robinson.

Photo 11. Sapwood decay and failure of a large scaffold branch on a Norway maple. Some of the fungi that colonize larger sapwood wounds can cause cankers and decay sapwood from the outside into the center of the stem, causing rapid stem strength loss and failure.

Photo 8. Sterile conk of Inonotus obliquus (aka Chaga) on a yellow birch. Note the cracking of the stem indicating extensive internal decay. The fungus only produces fertile fruiting after the trees fail or are removed, sometimes taking as long as seven years or more to sporulate.

Photo 9. Schizophyllum commune fruiting on a sunscald wound of a cut leaf weeping beech. The fungus colonizes damaged tissues, quickly decays sapwood and can kill bark and cambium once established.

Photo 10. Cerrena unicolor causing a canker and decay on a Norway maple stem. Note the wound wood on the edge of the canker and fungal fruiting in the face of the canker.

Photo 11. Sapwood decay and failure of a large scaffold branch on a Norway maple. Some of the fungi that colonize larger sapwood wounds can cause cankers and decay sapwood from the outside into the center of the stem, causing rapid stem strength loss and failure.