Just want to mention here that there is growing evidence that fully healthy trees can defend themselves against these diseases, even if they are labeled as susceptible.
For example, the apple scab fungus feeds on the amino acid arginine. An apple tree will naturally use arginine with the mineral cobalt, to produce complex proteins. A tree variety like Liberty is very good at this process, using up all of it's arginine, leaving nothing for the scab fungus to feed on. The scab fungus dies and Liberty is labeled as resistant or immune to scab.
A variety like Zestar however, does not take up enough cobalt to use up all of it's arginine. The scab fungus landing on it's leaves will find lots of arginine to feed on, and will thrive. Zestar is then labeled as susceptible to scab.
A company called Advancing Eco Agriculture has performed field tests on the effects of using a Cobalt foliar spray in orchards to prevent scab, and has had great results. After supplementing a variety like Zestar with cobalt, it then has the mineral needed to process all of it's arginine, and the scab fungus is then unable to survive.
I used their cobalt spray this past year. And saw no evidence of scab on my trees, whereas the year before I had a bad infestation on varieties like Zestar, Honeycrisp, and Wolf River. Foliar sprays do take a lot of work, so my long term plan is to fertilize my soil with Cobalt, and see if that will work as well as the spray when I get my soil up to the desired level.
I know they have also done lots of work with cherry orchards to eliminate canker, and have started working with fireblight in apple orchards as well. They have a product called Seacrop, which I'm told has been effective at preventing fireblight in early tests. Sea-Crop contains natural minersls and organic substances derived from sea water, and can be used as a foliar spray or liquid soil fertilizer. I have not used it.
It's my belief that if you fertilize your soil properly, so that you have the required levels of all of the minerals required by your trees, not just N, P, and K, your trees will be healthy enough to fight off all of the big 4 diseases, no matter the variety of tree. Not only that, but they should then be resistant to insects as well, and produce healthier fruit in the process.