Problem under wall strip batten

[Mee Mee]

My wall was mushy behind my bedroom door. Not wet-just mushy when pushed on. Removed the batten strip and found crumbling wallboard that doesn't meet together. How can I fill this in? Do I need to cut all the crumbling parts away and put in a new piece, then tape and mud? It runs about 50% of the way between the ceiling and floor. Help suggestions will be greatly appreciated!

[Greg]

It may be faster & easier to replace the wholr panel rather than try to fit a repair panel in place. As long as it is open you need to find the original problem and fix that also. It sounds to me like a leak. Greg