Chisago County Master Gardener
Heart rot can cause decay in both heartwood and sapwood.
Sapwood is living cells that move the sap through the tree, store extra energy, close off wounds, and actively fight invading microorganisms. In all trees, sapwood is in the outermost rings. Some trees, like maple, birch, beech and poplar, form only sapwood.
Other trees form heartwood at the core of their trunk. Heartwood is dead cells that serve primarily to add structural support to the tree, but they also contain a number of toxic chemicals that protect the heartwood from wood decay fungi.
How well these chemicals protect the heartwood varies from tree to tree. Cedar and redwood trees are so effective at defending against wood-rotting fungi that their lumber is highly valued.
You may have seen bracket fungus growing on rotting logs, dead trees, or living trees in damp woodlands. Many different fungi can cause heart rot. The interesting fungal structures that emerge in wet weather are spore-producing structures known as basidiocarps. If they are on a live tree, it indicates rot within the tree.
Heart rot fungi cannot infect a tree through intact bark. Instead, there must be a wound or opening in the bark, perhaps from a lawn mower, weed whip, fire scars, deer rubbing, rodent chewing, frost crack, broken branch or other injury.
Some heart rot fungi can also enter a tree through an old branch stub or through the tree’s roots.
Once inside the tree, the fungi use a variety of enzymes and other chemicals to break down the wood for food.
Trees often survive many years with heart rot. Because the tree can continue to grow, with no symptoms of disease or decline in the canopy, the problem can go unnoticed. The decay caused by many heart rot fungi progresses very slowly, often at a rate of about three inches a year.
A tree with signs of heart rot does not need to be removed immediately, but should be examined to determine how structurally sound it is. Give special attention to affected trees near people, or near property that could be damaged by falling branches or trees.