Aspen (?) leaf blight??

I live in central Newfoundland and have a sick tree. Last July all the leaves turned black and fell off. It developed new growth, though it was sparse. This spring the new growth appeared normal, if a little sparse, but now all the leaves are falling off again, and are yellowish with black/dark brown spots. Is this a bug or a mold or leaf blight? What can I do? The tree is almost 50 feet, much too big to spray. Thanks!

 

could be too much water on the roots

 

Such conditions are common, when you start to type aspen blight Google actually offers it as a possible phrase being looked for.

http://www.ext.colostate.edu/pubs/garden/02920.html