Experts and BHG readers answer.
"The yellow spots on the leaves of your apples are an indication of a fungal disease (quite likely cedar-apple rust). Sevin is an insecticide, and will have no effect on fungal diseases. For them, you need to spray a fungicide.
A 4-year-old dwarf apple tree is just at the beginning of its productive period. It usually takes at least 3-5 years for dwarf apple trees to begin bearing reliably. So you can expect greater yields in the future.
To grow apple trees as a fence (espalier), train the branches to grow laterally by tying them to wires when the branches are still flexible and non-woody. Remove the branches that grow outward in the opposite directions. Once the branches have become woody, they may no longer need the wire support."