Good Morning Alla, 

Thank you for contacting the U of M Extension Service. 

This may be insect galls. Please click on this link from the U of M Extension Yard and Garden website: https://extension.umn.edu/yard-and-garden-insects/insect-and-mite-galls 

I am unable to tell from the photo if they look like insect galls. Please review the above information and inspect the leaves carefully. If you scroll down, there is information on protecting your plants from galls. 

Please click on this link from the U of M Extension Yard and Garden website: https://extension.umn.edu/plant-diseases/apple-scab

This could be the beginning of apple scab. I would monitor the situation and evaluate the tree in the next few weeks. 

Good Luck!