Apples and especially young apple trees are kind of like chicken- everything wants to eat them! So how do you beat the deer, rabbits, voles, insects, fungus, and bacteria to it?
Mammals: There are probably as many techniques, sprays, and folk stories for dealing with each mammalian pest as there are people who grow apples, if not more.
In our experience, physical barriers work best. For deer, that means either a tall deer fence, or at the very least a roll of metal hardware cloth affixed to or buried in the ground around the base of each tree. A small cloth bag loaded with milorganite fertilizer, or another highly scented material associated with humans (soap, deodorant, etc.) will often work for a time, and has proven to be better than nothing in our orchard.
The same treatments can work for rabbits, but voles can be a little harder to exclude. Penn State University offers these helpful tips for dealing with voles: https://extension.psu.edu/orchard-wildlife-integrated-management-of-voles-in-orchards. Keeping the trunk clear of weeds, debris or mulch that can hide voles is always a good idea. A second layer of defense can be added in the form of gravel around the base of each trunk. Some farm supply stores carry gravel specifically meant to deter voles, and it is usually of a rougher grit.
Insects and Diseases: get on the email list for your state/local agricultural extension office. Even if you're not ready to spray, you'll learn the cycle of these pests and diseases over the course of a season, and knowing what to look for is half the battle. For help with identifying specific pests and symptoms of disease, the Tree Fruit Field Guide is an indispensable resource, and now there is a searchable online version, and it's free. The guide also lists beneficial insects, so start with learning what those look like, and follow up by finding out what the top pests and disease problems are in your area.