Hypoxylon Canker

I've noticed a surprisingly large number of hypoxylon cankers on trees during my walks in the Mont-Royal park this spring.

A hypoxylon canker is a fungal infection of a tree caused by one of a number of species of fungi in the genus Hypoxylon.

Hypoxylon canker

This fungal infection tends to take hold on a tree which is already stressed (weakened) in some other way [1]. Given that the infections I saw in the park were primarily in smaller trees, especially in the aspen family, which were adjacent to footpaths, I suspect that the trees were either suffering from wind/snow damage (aspens are brittle), light deprivation (from being overshaded by taller trees), or root compaction (from the passage of a large number of people over the season). Many of these individuals were also on a heavily eroded slope, so it is also possible that they were suffering from water deprivation or erosion around their roots. The fungal infection takes hold beneath the bark and, over the course of its growth, causes a bubbling and eventual sloughing off of the bark of the tree.

Hypoxylon canker

The cankers I'm seeing at this time of year are of course mature cankers, likely from last year; it is likely, given their black colour, that these stromata emitted their spores last year, which will infect new targets. The photos I have here show the fungus at the end of its life cyle, where the bark has sloughed off to reveal the 'bubbly' surface of the fungus, the spores have been emitted, and the fruiting body has hardened and darkened.

The best method to control this type of infection is to address the underlying cause of weakness in the tree.