How to spray large apple trees?

We have purchased a property with two large apple trees that I pruned the heck out in early Spring. My question is how do you spray these trees. They are about 15 to 18 feet tall and about 15 feet in diameter so they are pretty big. Is there an easy way to spray these trees? My back pack sprayer does not spray very far and takes a long time to spray these trees. I would like to stay on the ground and get good co verage. I was thinking of mixing up some fruit tree spray in a 5 gallon bucket and use a utiltiy pump to pump it into my high pressure washer and then spray the trees as this would prbably reach to the top of the tree. Anyone have any tips for spraying a large apple tree? Thanks for your help.