Apple Tree Diseases
This could be the first sign of cedar apple rust which is a fungal disease that needs both apple trees and cedar or juniper trees to complete its life cycle. Both types of tree must be present for the disease to spread, planted within 2 miles of each other. The first appearance on apple trees are small yellow spots on the leaves. Later, they enlarge and become red around the edges. The leaf undersides develops fringed, bumpy lesions. The appearance of these lesions as the spots enlarge is when you can confirm the identity of the affliction. This disease is not likely to kill your trees, but it can lead to early defoliation and stunted, misshapen fruit.
Here are some further resources on Cedar-Apple Rust: http://ohioline.osu.edu/factsheet/plpath-fru-02 and http://extension.psu.edu/plants/gardening/fphg/pome/diseases/rust
Some apple cultivars are rust resistant but Fuji and Honeycrisp are not.