Apple scab is caused by the fungus Venturia inaequalis. Planting disease resistant crabapple varieties is the best way to prevent apple scab. Many varieties of crabapple trees are resistant or completely immune to apple scab. If you replace your current crabapples with immune varieties then you should not need to spray at all. Cultural control is equally important. Remove fallen leaves in fall to get rid of places where the fungus can survive the winter to re-infect trees the next year. Fungus overwinter on fallen leaves. Even with good fall cleanup of fallen leaves, spores from nearby apple or crabapple trees could travel to your property, starting the infection cycle again. Prune crabapple trees in late winter so that the branches are spaced far enough apart to let air move through the branches and dry the leaves quickly. Remove upright suckers and water sprouts that have formed along the main trunk or within the canopy. Fungicides do not cure scab leaf spots but protect healthy leaves from becoming infected. For ornamental crabapple trees, fungicide sprays must be timed to protect new leaves as they emerge in spring. Sprays should start when the first green leaf tips appear in the spring( "½” green tip") Repeat sprays every 7-10 days until the majority of the flower petals have fallen off the tree. Fungicide labels give the recommended spray interval. In many landscapes, 2 well timed fungicides in spring will protect ornamental crabapples from apple scab. The number of fungicide treatments varies each year depending on the amount of rain during the bloom. Read more:

https://extension.umn.edu/plant-diseases/apple-scab#fungicides-1165363