Tree fertilization should be done after some soil samples are tested. The test will tell you what nutrients are lacking and how much, if any fertilizer is needed for the crop you're trying to grow. Test can be done by your local University Extension office.
Generally, fertilizing an apple tree will reduce fruit production.
As for the disease and pest management, I'm hesitant to recommend a plan because I'm not familiar with your area, but there is a product made by Ortho that is a combination fungicide and insect control for fruit trees. You can find it at your local garden center and the label will have more information.
Another popular method of insect control is bagging apples, which is just what it sounds like. You cover the fruit with a ziplock bag, put a staple on each side of the zipper to prevent it from coming open in the wind, and the bag blocks the bug from getting your fruit. They even make special bags for this. A google search will get you more info on that.
If you have Apple Scab in your area you'll need to spray fungicide 2 or 3 times in the spring. Scab will cause the leaves to turn splotchy and drop about mid-summer. It also leaves marks on the fruit.
If you can live with spots on your apples from scab, and take the time to bag them against the bugs, you can have organic apples, instead of having to worry about a bunch of chemicals.
