Hi,

 This looks like chlorosis, perhaps caused by our very wet weather. When soil is very wet it can make it difficult for plants to access certain nutrients in soil.

If the tree has been healthy until now, I wouldn’t worry about it too much. The issue may disappear when things dry out. 

If you want to try to fix it now, remedies are mentioned on the page below. Make sure you read all the links on the left side of the page. 

https://mortonarb.org/plant-and-protect/tree-plant-care/plant-care-resources/chlorosis/#overview

If the problem persists, you may want to do a soil test. They cost about $20 at the University of Minnesota Soil Testing Lab:

https://soiltest.cfans.umn.edu/

Good luck!