Apple scab has been widespread this year due to all the rain.  There is no treatment after the fact.  Any fungicides must be applied at 'green tip', that is when the leaves are just barely starting to appear in the spring.  For now, sanitation is important.  Remove all leaves as they fall and dispose of them.  Make your fall clean up very thorough.  This link will take you through the process.  At the bottom of the article is a list of applicable chemicals that are found in various fungicides and are valid.  Read the labels to see if Apple Scab is listed.  https://extension.umn.edu/plant-diseases/apple-scab#fungicides-to-protect-ornamental-crabapples-from-apple-scab-3652960