I have two large apple trees on my property that tend to suffer from scab and fly speck (which is a type of blight) every year. The fruit is pefectly edible, if not particularly pretty, but the disease hurts the crop yield. None of this kills the tree, but it affects the number of homemade apple pies and apple sauce I can churn out in the fall!
I have come across an organic apple tree spray which can be made cheaply and used on practically anything (I can't think of anything you COULDN'T spray it on!).
1 Cup of liquid seaweed with iron (you can order this online for less than $10 a quart and it will last all season). I use Maxicorp Liquid seaweed with Iron. You might be able to find this in your garden center. My local garden centers looked at me like I was from Mars when I asked. And forget about Home Depot or Lowes. You need a real gardners garden center
1 Cup of vinegar (any kind will do. I use apple cider but only because it is what I had on hand)
1 Cup Murphy's oil soap
I put all of this in a hose end sprayer and set the dial to 2 ounces per gallon. This spray mixture will last you a couple of sprayings, but you will need to spray WEEKLY.
If you aren't picky about organic, through into the mix a cup of regular old fruit tree spray (the kind with Captan). Or Bordeau Powder (which is organic).
The seaweed provides a follicular feeing. The vinegar helps control fungus and pest, and the oil soap helps keep the solution on the leaves and branches longer.
I will report back at the end of the season to let you know how it worked. So far so good. I am also using it to spray my black eyed Susan's which suffer from a black blight every year.
Apple trees they look so pretty
And their pies, they taste so sweet
But if you want organic apples
You must spray them every week!