Scab can affect your apple tree badly if you let it get a grip.
It's wise to always clear up the dead leaves at the end of the season, as the scab fungus will overwinter in the leaves.
If you have nasty scab areas on old wood, you can cut this out.
If you want you can spray with a fungicide, there are a few on the market, also Bordeaux mixture offers some control. Best time to spray is just before the leaves start to show in the spring, and repeat until the blossom starts to open.