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
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