Heart Rot
Phellinus igniarius and related decay fungi
A wood-decay disease in which a fungus rots the heartwood inside the trunk and large limbs of living trees such as alder, birch, and other hardwoods, hollowing the wood from the inside while the tree still leafs out. It rarely kills outright but weakens the wood, making the tree prone to breaking and degrading its timber value.
🔎 How to spot it
The outside sign is the conk, a hard, woody, hoof-shaped fruiting body that pushes out of the trunk or a branch stub, often gray to black and cracked on top. Inside, the rotted heartwood is soft and yellowish-white and stringy or spongy (a white rot), shot through with fine black zone lines that outline the decay column. A tree can carry extensive internal rot before any conk appears.
🥀 Damage it causes
The fungus breaks down the wood, so decayed trunks and limbs lose strength and snap in wind or under load, which is the real hazard in yard and street trees. The rot degrades or ruins the timber, and large decay columns can eventually contribute to the decline of the tree, though heart rot by itself usually does not quickly kill a vigorous tree.
🔬 What causes it
Decay fungi such as Phellinus igniarius, which infect through wounds, broken limbs, pruning cuts, and dead branch stubs and then spread through the heartwood. The conks release airborne spores that infect fresh wounds on other trees. Old, injured, and over-mature trees with many wounds are the most affected, and the fungus can keep decaying the wood for years.
🛡️ Prevent it
Because the rot is inside the wood there is no spray that reaches it, so prevention is about keeping trees sound and unwounded: avoid bark and trunk injuries, prune correctly so cuts heal cleanly, do not top trees, and keep them vigorous. Remove dead and broken branches promptly before they become entry courts for decay.
🧯 If it is already here
There is no cure for wood already rotted. Management is to assess and reduce hazard: have a heavily conked or hollow tree evaluated, remove dead and damaged limbs, and take down trees that have become structurally unsafe near targets. Conserving tree vigor and avoiding new wounds slows further decay.
💡 Good to know
A hoof-shaped conk on the trunk signals substantial internal rot even when the canopy still looks healthy, so it is a cue to check the tree for safety. Since the fungus enters through wounds, careful pruning and avoiding trunk injury are the best long-term defense.
🌱 Plants it affects
17 plants in the library can be affected by this problem
For educational and informational purposes only. Disease management advice is general guidance drawn from university cooperative extension sources; always identify a problem positively and read and follow the label on any product before use, especially around food crops, children, and pets.