AbstractDieback-decline diseases occur when trees, stressed and altered by abiotic or biotic agents, are attacked by organisms of secondary action. The primary stress factors in forests are insect defoliation and extremes of moisture and temperature. In urban situations, drought is probably the most important stress factor. Most organisms of secondary action are native opportunists which often are more successful in urban than in forest situations — perhaps because urban trees are more frequently subjected to prolonged or intense stress. Attempts to control these diseases usually should be focused on preventing or reducing stress effects rather than on direct actions against the secondary organisms.