When and how to prune young apple trees
This is a great time of year to do pruning of fruit trees. We have a detailed article available at http://cmg.colostate.edu/Gardennotes/771.pdf We also have a shorter overview of pruning fruit trees available at https://extension.colostate.edu/topic-areas/yard-garden/training-and-pruning-fruit-trees-7-003/
Often when apple trees don't have fruit, they either did not get enough pollinator activity, or more likely they reached their critical temperature in the spring. Often we get light frosts in the spring which nips our developing fruit causing them to abort or fail. Poncha and Salida had good fruit set last year, BV had pockets of damage in 2018.
I hope this is helpful!
Thanks for the helpful info Kurt