Hypoxylon canker on oak (Quercus): note the white cushion indicating the presence of a fungal mat
White fungal mats of hypoxylon are visible through the splitting bark on an oak (Quercus)
Hypoxylon canker on shingle oak (Quercus imbricaria): note the area of sloughing bark on the top, silvery area of fungus underneath, and fruiting bodies on the bottom.
Fruiting Bodies of Hypoxylon. USDA Forest Service - Region 8 - Southern , USDA Forest Service, Bugwood.org
Hypoxylon canker is caused by an opportunistic fungi, Biscogniauxia (formerly Hypoxylon) atropunctatum. Hypoxylon is unable to cause disease in healthy trees but is quick to colonize weakened or dying bark and wood. Oaks in the black oak group (pin, red, black, scarlet, and willow oak) are more susceptible than those in the white oak group (burr, chestnut, swamp white, and white oak). The disease is also found as a saprophyte (living on dead wood) on basswood, beech, hickory, hornbeam, maple, and sycamore.
Symptoms and Diagnosis
Yellowing and wilting leaves may signal the onset of the disease, but these symptoms are merely general indicators that the plant is under stress and are not necessarily caused by the fungus. The most recognizable sign of a problem is the sloughing of patches of bark on the branches or trunk which exposes a silvery or cushiony layer of fungus. Prior to the bark sloughing, sunken or depressed areas appear in infected areas where the fungus has killed the cambium. Dead or dying limbs in the tree may also be the result of hypoxylon canker.
The fungus enters the tree through wounds on the branches or trunk and grows in the sapwood where it kills this conductive tissue. There is evidence that the pathogen may invade young trees but not cause a problem until the tree is under stress. Infected branches progressively die back as the fungus moves downward. Dead, sunken patches develop on larger trunks where the advancing fungus kills the conducting sapwood. Trees may die in one to two years or sooner if the early stages of the disease go unrecognized.
Integrated Pest Management Strategies
1. There is no effective control for the disease. If over 15% of the crown is infected, cut the tree to ground level and burn it, or have a hauler remove it from the site. Since the fungus can remain active in dead wood, do not chip the wood and use it for mulch. Also, any wood burned as fuel should be used quickly.
2. If 15% or less of the crown is infected, prune out all infected branches and cankers. Dispose of as noted above. Also, improve the health of the tree by watering and fertilizing.
Strategy 1 is a strictly organic approach. Using an appropriate organic fertilizer would be a viable organic approach to Strategy 2