The best treatment is to cut it out but that does not look possible. You can spray copper any time you use less when the tree is growing. Copper is best as a preventative, but I would spray it. More important spray your other tree! You can wait till the fall to do it. As once leaves fall you can up the dose. The best copper is Kocide, look at our spray amounts guide for how much to use. Lime-sulfur is another decent product, but copper is better for canker. You won’t cure it, but the tree should live many years with it. And infection can stay local. Copper can be used on many plants for various problems, so it is a useful spray to have. Try it on powdery mildew on anything or blight, septoria, on tomatoes etc.
Here is the spray amounts guide. Kocide only lists per acre, so it is useful!