Prune any dead or diseased wood out of the tree right now. Don't worry about percentages, get it out of there. Fire blight looks like blackened bark, and can be spread to healthy wood by your pruning tools, so if you have fire blight, haul along a 5 gallon bucket of strong bleach solution to disinfect your tools.
Besides pruning, apples really benefit from a dormant spray made of lime-sulfur and a little oil. Hit the tree with a strong spray before the buds swell or you prune, then follow with a couple more dilute sprays ending at blossom.
Worms in apples burrow from the core out. They drop to the ground under the tree and pupate until the next blossom, when they hatch to moths or flies and fly up into the tree to lay eggs in the blossom. Most fruit parasites are not good at flying, so you can really reduce your problems with good orchard sanitation.
You will see professional orchards that are bare dirt under the trees. This disrupts the fruit fly reproduction cycle, and also removes competition from grasses for moisture and nutrients. Roundup under the tree before the buds swell, and rototill under the tree before blossom. You don't need to till deep, just deep enough to get the grass roots.