I'm not sure if you can... I would try these to fix it.
-cast dispell magic on the tree
Or
-Have a hidden dummy harvester with the gather lumber from NE,Orc or Hu. Order it to harvest the tree for an instant and see if it corrects. (Ghouls "blight" trees upon hit, so maybe the reverse is true)
Or
-Simply replace the tree, make sure the ground is not blighted or else it may re-blight it.