Hypoxylon canker is a secondary disease of many hardwood species that affects trees that are already severely stressed by some other cause. There are many Hypoxylon fungi, most of which are strictly wood-rotters, but some species can become pathogenic and kill stressed trees. The disease is most common in large mature trees, especially after prolonged periods of severe drought. Hypoxylon canker is a common contributor to the oak decline disease complex. (Note: there is a separate, unrelated disease of aspen also called Hypoxylon Canker).
Symptoms may initially resemble those of oak decline: delayed bud break; undersized leaves; chlorotic, scorched, or wilted foliage; and branch dieback progressing from the top of the tree downward. The easiest way to identify Hypoxylon canker is by the large spore-bearing mats (stromata) produced beneath the bark of diseased trees. These stromata usually appear the year following drought (or other severe stress), but may appear within a few months. Stromata will grow in size and eventually rupture the bark; patches of sloughed-off bark range from a few inches to several feet in long. In severe cases, almost the entire tree will lose its bark and be covered in the fungal mats. Stromata are initially tan, brown, or black; after all of the spores are released the mats turn grey. Trees die quickly, if not already dead, shortly after stromata production.
Few management options are available.
What Can You Do?
Proper tree care is critical. Provide trees with adequate water (at least 1-2 inches per week below the entire drip line), fertilizer if necessary (avoid nitrogen-rich fertilizers), and room to grow with little competition from neighboring trees or understory. Provide a thin layer of mulch (1-3 inches) below as much of the drip line as possible. Avoid planting susceptible species on dry sites.