We need more information.
Where are you?
Did you plant this tree? Is it near a sprinkler system or in a low area? Do you know about the problem of planting trees too deep in the ground? http://www.dirtdoctor.com/Tree-Newsletter-Planting-Series_vq1941.htm
Are you an organic gardener? Did anyone put out a weed-and-feed mix in your yard this year? That stuff is toxic, and if you read the package, it says not to put it near trees.
Anything else you can send, and if you can post of photo, would be helpful.