Thanks for your question. Zestar apple trees have good resistance to mildew but are susceptible to scab. Given the photo and description apple scab seems the culprit. UMN has good information on its management that I think you will find helpful. Generally, for trees with scab, collect and discard (do not compost) fallen, infected leaves  in the fall. Leaves are a major source of spores that can infect the tree the following growing season. Also, routinely thin your tree, opening up the canopy to improve air circulation. Thinning promotes faster drying of leaves which in turn are less favorable for scab development. Even with proper fall clean up and thinning fungicide treatment may be needed. If go that route, be sure to read and follow all label instructions to ensure treatment is used in the most safe and effective manner possible. Good luck!