caring for apple trees
The short answer is that it depends on what issues the apples have suffered in the past. Fruit trees are best managed with prevention in mind, as various treatments safe to use on edible crops are meant to be used as prophylactics rather than curatives. Once a fungal or bacterial disease is detected, it's often too late to effectively treat for that growing season. General sanitation is important: cleaning pruning tools between trees (to minimize the risk of spreading spores), pruning off dead wood, and removing infected foliage that dropped during summer or fall to minimize exposure to a bank of spores ready to re-infect next season. Plant diseases are also weather-dependent, so differing conditions year-to-year can result in heavier or lighter disease pressure. Stressed and wounded plants are also more susceptible to infection and infestation, so minimize this by irrigating plants during drought, fertilizing only when needed (using a soil test is best), and protecting the bark from damage due to lawn equipment, chewing animals, and buck rub (male deer rubbing velvet off their antlers in fall). Similarly, make sure there are no stakes whose ties to the trunk are cutting into or abrading the bark. Avoiding wounding is particularly advisable for preventing bacterial diseases, which are not easily treated (if at all); bacterial infections typically take advantage of plant wounds to be able to infect the plant, as opposed to fungi, which can make their own entry point.
If you had an insect attack last year, knowing the culprit's identify will help in determining the best course of preventative treatment this year. Some insects emerge from winter dormancy early in spring; others much later. By learning the life cycle, you can avoid unnecessary treatments and most effectively time them to your target pest; otherwise, poorly-timed applications may harm beneficial organisms and waste money. In general, insecticidal oil and soap sprays are some of the most benign treatments meant to smother overwintering insects if they are exposed on the bark. These treatments may also carry the side benefit of smothering disease spores as well, but again, only if they are on the surface of the bark. Similarly, there are a few surface-treatment fungicides that can control surface-dwelling spores of fungal (and some bacterial) organisms; timing and frequency of sprays will depend on the specific product. Some are labeled organic and others not; verify the label instructions list edible crop plants such as apples are included and how late the treatments can be applied before harvest time. Weather plays a role in spray effectiveness and timing: pest and disease activity are as tied to the weather as when the apples will break dormancy, and so may not be applied at exactly the same time from year to year. Some disease and insect overwintering stages are already in the interior of the plant where they are difficult or impossible to reach with sprays.
There are some products used as systemics - absorbed by the plant, usually via roots, and moved internally to other tissues like leaves - but there are limited options able to be used safely when treating cropping plants. Systemics are the only treatments that might reach certain insects and diseases, and since many systemic products are not labeled for uses on edibles, this is why prevention is best.
This page gives information on fruit tree pest and disease control for our area: https://extension.umd.edu/hgic/topics/fruit-disease-and-insect-pest-control and this page https://extension.umd.edu/hgic/topics/apples lists some apple care information and troubleshooting. For best tree nutrition, you can have a soil test done (information on how and where is here: https://extension.umd.edu/hgic/topics/soil-testing) so you know if any nutrient levels or pH needs correcting. Cropping plants tend to need more supplemental nourishment than other garden plants, but too much can still be a hindrance and a pollution source. Winter would be a good time to send samples in so you have results to work with come spring.