Is there an insect or blight affecting poplar trees this year?
Without seeing the leaves it is difficult to diagnose the problem. Here is a link to a very helpful bulletin on leaf spot diseases from University of Minnesota Extension: Management of Leaf Spot Diseases of Trees and Shrubs. The bulletin includes photos that might help you identify the disease and gives treatment options.
Bacterial leaf spot is known to create a shot hole appearance. According to an excerpt from the bulletin, "Bacterial leaf spot diseases often start as small dark brown to black spots with a halo of yellow tissue surrounding each spot. In some bacterial leaf spot diseases, the center of the leaf spot will dry up and fall out, giving the leaf a 'shot hole' appearance. If weather conditions remain favorable for disease, some bacterial leaf spots will grow together creating large black blotches on leaves or turning leaves completely black. Shoots, buds and occasionally flowers can also become black and blighted by bacterial leaf spot pathogens."