Fungus on cherry tree
Your cherry tree has rot in the main trunk and it may hang on for a few years. It is sad but cherry trees last only about 15 years some do not last that long.
Will it help the tree to cut out affected area and tar the spot?
No the rot is inside. Tarring over wounds has not been recommended by University experts for more than 20 years because studies have shown it causes more fungal problems.