One thing that would help for starters is to prune and remove the dead and decayed limbs. You should research how to prune old apple trees first, its got to be done right or they might not produce for a year or two. Pruning should be done late winter/early spring. Removing competing brush and trees would also be a big help.
I've always used Bonide Fruit Tree spray starting right before the tree is coming out of dormancy. The first early spraying is the most important, but frequent applications through out spring and summer is required for best results.
