Heart rot is caused by fungi entering the trunk of the tree through wounds in the bark. These wounds are areas of the tree where bare wood is exposed and usually a result of improper pruning, fire damage, dead branches, insects or even animal damage. The fungal spores enter the exposed wounds, germinate within the wood tissues and slowly ingest the heartwood. The infection is a very slow process and can take from months to years, depending on conditions and tree health. An average fungus will grow 6–8 centimeters per year, and extensive development in the wood tissue is needed to produce mushrooms or conks.