Backpack sprayers for apple orchard
 

I have an orchard with old, tall apple trees, some 35 feet high. We
are gradually pruning them back, but severe pruning over the last few
years has limited our crop, together with alternating drought and
excessive rain. I also have apple scab. Anyway, I need a powerful
sprayer to spray up over the canopy of leaves, with
insecticide/fungicide. Some backpack sprayers go up to 180 psi others
90 psi. Some say a lower pressure gives a larger droplet size and
less dispersion. Some pumps are diaphragmatic, others are different.
I'm confused. Any recommendations welcome. Also if anyone can
recommend a product for apple scab I'd appreciate it.
Jay