Several causes for Dieback - Various fungi with some invading healthy stems, some entering the plant through wound sites or other points of damage, poor cultural practices - poor establishment of young plants, drought and waterlogging, also through wind burn and frost damage. Cut out dead wood below area affected. Dig up and remove whole plants if necessary and dispose of them correctly.
Product Solution: Paint cuts and prune sites with Arbrex Seal & Heal. Ensure that plants are well fed and that drainage is adequate.