This is the quintessential target canker of hardwoods, often with nice, fat, concentric annual rolls of callus. Like a pleasant work of art in the forest.
Hosts include many hardwoods.
The pathogen was once in the genus Nectria but is no longer. Like so many other diseases named after pathogens, this renders the disease name awkard and meaningless. The pathogen is Neonectria ditissima (often called Nectria galligena in older literature). It was found in North America as early as 1897 and is very likely native .
Infection appears to take place via small dead branch stubs. The pathogen kills a patch of bark. The inner bark dries and cracks, and the bark soon falls off. Then the magic begins:
- The host produces a callus roll around the canker. If successful, this would continue growth each year until it covered the dead area and sealed it.
- During the dormant season, when the host can’t actively respond, the fungus kills said canker roll.
- When the tree awakens next year, it finds itself taken aback. Not knowing what else to do, it lays down a phellogen barrier between live and dead bark, then produces another roll of callus around the dead one.
- Get where we’re going with this? The second roll gets killed as well. Back and forth the combatants go, creating this work of art that takes years to develop.
The canker persists for many years without killing its host.
The canker face is usually free of bark, making it rather attractive, but it may have bark on aspen.
We rarely see fruiting of the pathogen. It probably fruits mostly in certain years when weather is right, and then in the dormant season when we’re not around to see it.
Eutypella canker is a persistent perennial canker and an important disease of sugar maple in the midwestern and northeastern United States and in southern Ontario and Quebec.
The host range is primarily restricted to maples. Acer saccharum (sugar maple) is infected more commonly than other native species. However, in North American urban areas, it can be quite common in Acer platanoides, a species native to Eurasia .
The pathogen, Eutypella parasitica, produces black perithecia embedded and slightly protruding from bark on old cankers (see featured image above). This gives the bark a blackened appearance. Perithecia are produced on parts of the canker older than 4-5 years, or may be confined to the center of the canker . Thick, white to buff mycelia fans are produced beneath the bark.
Infections typically occur through dead and dying branch stubs, usually small ones . The stub often persists at the center of the canker face. A smaller number of infections occur via wounds.
The disease often is most frequent in stands that have had some type of partial cutting . Increased light after cutting stimulates epicormic branching on the stems. Epicormic branches frequently die after the crown closes and they receive less light. The pathogen is thought to invade these dead or moribund branches, or their stubs, then progress into the stem and initiate a canker. This is consistent with the fact that a dead branch stubs is usually present at the canker center
Small trees, less than ~12 cm DBH, can be killed directly by the disease. Larger trees may snap due to distortion associated with the canker and/or wood decay behind the canker. The pathogen itself penetrates the wood and causes some wood decay  and other decay fungi may infect the stem through the canker. Cankers tend to occur low on the stem. Sixty percent of cankers occur in the lower 2.5 m . Thus, snapping at such a canker would usually be fatal.
Canker margins grow approximately 1.3 cm per year , but faster vertically than horizontally . Callus is produced by the host in response to the canker, but largely on the sides rather than the top and bottom. This accounts for the flaring, cobra-like margin.
It is interesting that cankers can be rendered completely inactive by excision of the diseased bark [2, 3]. It is unknown whether the fungus remains active in the wood.
In summary, Eutypella canker is characterized by these features, which also help differentiate it from Nectria canker:
- Practically restricted to maples.
- The target appearance is usually not so clear.
- Cankers often have flaring lateral margins, like a cobra.
- The bark generally remains attached over the dead canker face.
- Fruiting can usually be found, black perithecia embedded in the bark partially exposed in the centers of old cankers.
- Wood becomes discolored deep into the stem, and eventually becomes slightly decayed.
- Under the bark at the advancing margin of an active canker, white to buff mycelial fans can be found.