Heart rot in living trees is caused by fungi which have entered the tree through open wounds and exposed bare wood. Usually a conk or mushroom "fruiting" body is the first sign of infection. All deciduous trees can get heart rot.
A useful rule of thumb might be that a cubic foot of wood has decayed for each conk produced. Heart rot fungi do not invade living wood of healthy trees.
As long as a tree is growing vigorously, rot will be confined to a small central core. This is called compartmentalization. If the tree is weakened and fresh wood exposed by severe pruning or storm damage, decay fungi can advance to more and more wood. Keep your tree health
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Minimize pruning wounds that expose large areas of wood. Shape trees at an early age so major branch removal will not be necessary later. Remove broken branch stubs following storm damage. Have suspect heart rot trees checked by an arborist to determine if sufficient live wood is present for structural safety. Check trees every few years to be certain new growth is maintaining sound structure. Large trunks and main branches with extensive decay may have little sound wood to support the tree.