Hello,

Your poplars look like silver poplar, Populus alba, or one of the hybrids. They are related to cottonwoods and aspens. The leaf spotting looks like Septoria, or possibly 

Shoot blight. The root zone may be crowded, compacted or otherwise stressing the tree and causing some dieback.

Here is an article describing leaf spots on Populus species trees

https://extension.colostate.edu/docs/pubs/garden/02920.pdf

I would do one of two things: 

Submit samples to a lab for diagnosis- you

may send leaf and stem samples to MSU Plant and Pest Diagnostics lab. Samples should have still green leaves and some leaves that show yellowing. Include pictures of the whole plant and a close up of the problem. 

See the web site for fee schedule and instructions- 

https://www.canr.msu.edu/pestid/

Or hire a certified arborist, a professional who has taken training in care, diseases, pests and passed certification tests. He/she will come on site and give a complete diagnosis, examine the root zone, and a plant care plan. Find certified arborists by zip code here—-

www.treesaregood.org

In any case, follow the advice in the first link to help the trees as much as possible. And keep them watered during very dry times. Thanks for using our service.