I have a 15 year old flowering crab apple that is not doing well. Last year the tree bloomed beautifully, then not long after the leaves started turning yellow and falling off.

Now the same thing is happening. I don't see any obvious bugs, we have never sprayed the tree with chemicals. What do you think could be the problem and what can I do? I just walked out on the deck and there are yellow leaves all over the place. I live in the Chicago area.

thanks