Since September, a typically rare disease known as hypoxylon canker has been showing up much more frequently on our urban trees than in the past 13 years I have been practicing arboriculture. Red Oaks, Live Oaks, and Post Oaks, already weakened by this summer’s harsh drought and heat, are falling victim at an alarming rate. It’s important for us, as responsible citizens, to recognize and work to prevent hypoxylon canker, not just to preserve the beauty of our urban landscape but also to help prevent our trees from becoming hazards.

Perhaps the sole silver lining of Hypoxylon canker lies in its distinct and recognizable signs. On our local Oaks, this disease manifests as patches of grey, tan, or brown lesions on the bark of the stem and branches. As the bark peels away, it reveals the underlying fungal infestation. On Sycamore and Pecan trees, Hypoxylon canker presents as black patches, usually confined to dead or dying wood. Prompt removal of these infected branches is crucial to halt the spread of the fungus and protect the tree’s vital stem wood.

Hypoxylon canker serves as a grim signal of the decline and death of an Oak tree. Typically triggered by severe stress (typically drought, compaction, or construction damage in our region) it preys on weakened trees, swiftly infecting them and quickly leading to their death.

The appearance of hypoxylon canker on a branch or limb is a near-certain sign that the branch or limb will die and need to be removed. When the canker spreads to a tree’s stem or trunk, it almost always spells the tree’s end. Once hypoxylon manifests visually, intervention is often futile. In exceptional cases, prompt and aggressive care with water, mulch, and fertilizer might save the tree, but usually, such efforts are in vain.

Quickly removing trees afflicted with hypoxylon is crucial. In its advanced stages, the disease causes white rot, drastically and quickly reducing the tree’s structural integrity and posing a significant hazard. The removal of a tree severely weakened by advanced hypoxylon is not only dangerous but also typically more costly due to the increased risks.

However, if the infection has compromised less than 15% of the canopy, there is a glimmer of hope. Pruning the dead branches and rejuvenating the tree through soil decompaction, mulching, and regular watering might just turn the tide against this devastating disease.

Hypoxylon canker, a fungal disease, is usually already present as spores in the bark many of our local oaks, pecans, and sycamores, but it typically only infects and becomes harmful when the tree is under severe stress. The most effective strategy to prevent this disease from damaging your trees is straightforward: maintain a consistent 3-4 inch layer of hardwood mulch covering the root zone, and ensure regular watering during drought periods. By keeping your trees strong and stress-free, you greatly reduce the risk of hypoxylon infection, safeguarding the health and longevity of these vital members of our urban and natural landscapes.

Hypoxylon Canker is a devastating disease that quickly follows severe stress, rapidly killing urban trees like Oaks, identifiable by distinct bark patches and often fatal once your tree is infected. Infected trees should be removed quickly as the disease makes the tree hazardous very quickly. Prevention involves regular tree care, including mulching and watering, to strengthen trees against this deadly threat.

The Death of Urban Trees 1
The Death of Urban Trees 2