Edwin,

We have a similar problem with our 30 year old crabapple in our front yard.  I read and questioned everyone I knew on how to stop the apple scab, but usually ended up with the same answers: (1) once your tree gets infected it is nearly impossible to get rid of it, debris maintenance and timely spraying will help, (2) it is not usually fatal to the tree unless you get total defoliation and lastly (3) if 1 and 2 are not acceptable, your best bet is to remove the tree and replace it with one of the newer varieties resistant to apple scab.

  Here is a link to an article from Michigan State University Extension on apple scab of crabapple trees and it is the best discussion I have found: https://www.canr.msu.edu/news/why_is_my_crabapple_tree_losing_leaves

   It covers the topic of apple scab quite thoroughly and includes additional links to the University of Minnesota Extension on managing apple scab and from Purdue University Extension providing a list of apple scab resistant varieties of crabapple trees.  These are especially good sources of information in that they are reasonably current and they are from sources in our region.

  I'm sorry I don't have a simple fix for your apple scab problem and it's difficult to remove a tree, especially if it is still alive and providing beautiful blossoms each spring. However, after years of procrastination, I plan to have our crabapple removed and replaced with an apple scab resistant variety this fall. Just like you, I have tried everything, but ours is in the front of our house and that has become our primary decision maker.

  I wish you the best for your crabapple tree and I hope the link I provided helps give you the information you need. Also, I hope you have a great summer and hopefully, soon we will get out of this rainy weather. Probably won't be long though before we are praying for just a little rain to break the long days of hot and humid weather of July and August.