Like everybody, I have some fruit trees in the backyard. Ontario made my life miserable by banning all the horribly toxic stuff I used to pour on them. So I had to use all my chemistry to mix up something as good. Now, you know that I never put any of this stuff on my trees, since that would be illegal. I am just coming up with a 'thought experiment' on what would be good, and has worked the last few years.
I just thought of 2 ingredients - one is a wettable powder organic fungicide. I use Bordo Copper, since I figure that if I would accidently spray myself, it would prevent arthritis :). The powder is great, since it has a wetting agent, and this is legal. Next I put in some pyrethin, which is organic, but banned because my fruit trees may be on a swamp. I would use Home Defence, if I actually sprayed it. I put in about 60 ml for 3 l. You have to spray every few weeks.
And here's something obvious, which I didn't know for years. When the apples are about an inch in diameter, pinch out all the apples in a cluster, but the main, biggest one. It really helps!