The leaves of my plum tree are turning black and seem to be falling earlier than usual. I recently had a fence put up and the roots may have been disturbed but I wouldn't think that that would cause a problem.
Plum Leaves Drop Early
Certified GKH Gardening Expert
It sounds like you have a fungus. Disturbing the roots probably did not have a direct effect, but it may have weakened the tree a little, which made it more susceptible to fungal infestation. Treat the tree with a fungicide and this should clear up the leaf drop and the black leaves.