Verticillium WiltThis disease is a soil borne pathogen that enters a trees vascular system and as it develops it obstructs internal movement of water and nutrients in the tree.  This causes gradual weakening and dieback of branches, known as flagging. A cross section sample of an infected branch shows a green discoloration. Treatment is impossible since no cure has been found. Maintaining plant vigor through deep root fertilization can give positive results if decline is not too severe before treatment.