It appears that it may be one of the many leaf spot diseases that affect poplar and similar trees.  I cannot identify it from the leaves in your picture.  I have included below, several links.  One that will tell you more about leaf spot diseases and important management information. Another is a link to hybrid poplar. I don't know which cultivar you have and the link describes several and details its environmental requirements and pests. <personal data hidden> is a community and urban forester located in Sioux Falls; perhaps you might email him and see if there is anyone in your area he can suggest to assess your tree.

  https://www.ag.ndsu.edu/trees/handbook/th-3-133.pdf

https://extension.umn.edu/plant-diseases/leaf-spot-diseases-trees-and-shrubs#identific