What wold be a good liquid foliage spray to use on apple trees?
It is always a good idea to know what you are spraying for before you start spraying. Apples have a variety of pests, ranging from diseases such as scab and fireblight to insects like plum curculio, coddling moth and apple maggot. So you first must decide what you want to spray for.
Having said that, the most common and destructive apple pest is apple maggot. Chemical control for this insect begins toward the end of June and extends all through the summer. Unfortunately, many of the most efficacious insecticides are not available for home use, or only at a rate that provides minimal control.
Instead, we strongly recommend bagging apples for maggot protection. It is easier over the long term and does not harm non-target insects. Read here about apple maggots and their control, including bagging:
This publication gives information on controls for other apple pests: