My peach tree trunk got severely damaged by a big branch breaking off from it. Do I need to do anything to save the trunk or tree by applying something to heal the scar and prevent disease that might weaken the fruit tree. Would appreciate any suggestions to save the tree trunk.
That stub of heartwood projecting out at the bottom of the wound is trapping rainwater and debris which supports decay. I would trim it off so that nothing can collect in that pocket and so it dries quickly. You can see the puckered scar tissue around the edge nearly all the way around except where that stub has been left jutted out.
The first step after damage like that is always to cut away any area that can never heal.
Related to this sort of damage is prevention, which is largely a matter of seasonal pruning. Orchardists spot sharp V shaped joints between branches or branch/trunk and remove them when young as the wood will always be prone to damage like yours.
U shaped joints are preferred and the overall weight of wood with fruit determines whether a branch must be trimmed or not. Included in this decision is the method of harvesting and balancing sunlight among the branches.