When mold and decay occur, the problem is either water conducting dryrot or dryrot growing on wet wood. Water conducting dryrot feels leathery and can often be peeled off in sheets. The other form of dryrot feels powdery or stringy. The only way to get rid of molds and decay fungi embedded in wood is to remove the piece. Frequently extracting a rotted structural piece can be an expensive proposition.
Prevention is far cheaper.
A common source of dryrot is wood decks. Decks themselves should be made of either cedar, redwood, sunwood or pressure treated lumber which all resist dryrot. The deck should have positive drainage away from the building. The deck door should have proper flashing to prevent water intrusion. Windows and doors are a prime location for dryrot due to improper flashing and caulking.