Thank you for reaching out to ask2.extension.  I found this information from Iowa State University. The main objective in spraying apple trees is to prevent insect damage to the fruit. The most important period to spray apple trees is from petal drop until just prior to harvest. Several applications will need to be made during this period. A home orchard type spray (available at most garden centers) is the best product for home gardeners. Most home orchard type sprays contain 2 insecticides and 1 fungicide and can be applied to apple, pear, and most other fruit trees. See the product label for specific directions.

To learn more about specific types of diseases and what to treat with google Pest management for the home orchard from conservancy.umn.edu.

As a volunteer I can’t give you specific chemicals to use.

I hope this helps!