Thyronectria fungus on a honey locust; how to treat?

The tree guys tell us that our declining honey locust tree has Thyronectria fungus,  mad worse by drought. Besides watering, how would you go about treating the tree? He wants to apply Cambistat to prevent the tree from growing for a few years, so that it can focus on healing; is this a good idea?

Can you get some cold pressed neem oil and apply to the area where the fungus is?