Don't know if all the rain is the cause but the Maples are loosing alot of 1/2-1/3 grown leaves which have turned mostly yellow.Trees appear healthy otherwise.
Sugar Maples
Individual branches or spread throughout the tree, Peter?
Victor,I would say off hand individual branches but I will look more closely when I get home.
In that case, you might have to consider verticillium wilt. It killed my beautiful large JM this last year.
http://www.treehelp.com/trees/maple/maple-diseases-maple-verticillium-wilt.asp
Did more inspecting and a good part of the tree looks normal but branches scattered are yellowing .
Perhaps there is some kind of borer in those branches? You could tell from a hole somewhere along the branch and "sawdust" somewhere around it. In that case, I would cut off those branches, making sure you cut below the borer's entrance.
Another possibility: do you have any Black Walnut trees nearby? We had a bunch of formerly healthy trees die off near the edges of our black walnut planting after heavy rains, which apparently spread the root toxins they exude to a wider area.
If neither of the above is the case, perhaps the verticillium wilt mentioned by victorgardener is the culprit. Or maybe there's just been so much rain that the tree is sitting in groundwater, and roots are drowning.
Hope these thoughts help. Good luck.
Many thanks to all