We planted a plum tree in the spring. Can you tell us what is going on with...
In your photo it looks like the tree sustained some damage to the trunk. We cannot say if it was mechanical, wildlife, etc. It is possible that there may be a canker or cankers on the trunk that can cause dieback. There are no controls for cankers other than pruning and this is not possible on the main trunk. See our website for more information and photos. http://extension.umd.edu/learn/cankers-trees-and-shrubs
At this point, there is not much you can do, except keep watered during dry periods, Make sure mulch is no thicker than several inches and keep away from the base of the trunk. All you can do is monitor the tree for more symptoms. The tree looks young and you may want to consider replacement.