Maybe the info at the link will help you identify the cause.

Here is a link that might be useful: Poplar insects and diseases

The only visible problem with my dying poplar tree is that the leaves look wilted and are rolled up along the edges. There is no sign of gall, no oozing sap, nothing except the leaves and dying limbs. There are other poplar trees in the neghborhood (in fact developer put them on the lines between every lot) and the other poplar trees don't seem to be having a problem. I moved in house in August and the tree seemed to be fine. I really didn't have a reason to examine it closely but it could have had some dying twigs then. Right now with it hot and dry, I am watering it as the leaves indicate lack of moisture to me. Thanks for any help.