It could be a fungus or mold.............cut off the effected leaves, in the FALL...........do dormant oil.............and in the spring up to bud stage............when the buds open........stop...........just spray the trunk. Also it needs to be trimmed in the spring we also add very old fertilizer and check for the PH...........it should be 6.0-8.0...it might need lime, we have to do that every spring also....................You can put some mulch around it after all this is done to keep moisture in for the growing season but it the fall rake it away from the tree or you will have mice that will nest & dig into the roots and cause a problem.........do dormant oil again when the leaves have dropped...... Happy Spring. When you water only water at the root base.......