The first signs of this disease can often be found on the under the surface of the leaves as they emerge from the buds in the spring.
Apple scab spores are blown around in the air and land on the under surface of leaves.
As the leaves continue to grow, both surfaces can be infected as can the fruit.
The signs to look for on leaves are velvety, brownish, small circles.
As the infection takes hold, these lesions start to grow together and the surfaces of leaves and fruit become distorted.
Keeping scab infection to a minimum begins with raking and removing leaves from under the tree the previous fall. Planting varieties that are resistant to scab is another way to minimize infection. William’s Pride, Freedom, and Liberty are immune to this disease. Honeycrisp has some immunity as well.
If the variety you plant is not immune and you see signs of scab early in the season, the best way to protect the fruit is by covering it with a plastic bag or applying a well-timed spray of organic fungicides such as lime sulfur.
Information source: https://extension.umn.edu