23 - Diebacks and Declines—Complex Diseases

Abstract

There are many cases where the cause of the decline or death of trees cannot be attributed to only one agent. In these cases, primary stress or stresses cause(s) the health of the tree to deteriorate and any number of secondary pathogens or insects may cause the eventual decline and death of the tree. These diseases are called “diebacks or declines,” or “complex diseases.” Complex diseases can be understood by comparing them to diseases with single primary causes such as Dutch elm disease or white pine blister rust. In the diseases with single primary causes, the condition of the host and the environment play only a relatively minor role in the disease. However, in dieback and decline diseases, the host condition as altered by adverse environmental factors is of major importance. The organisms of secondary action are able to succeed only when trees have been altered by stress. Dieback and decline diseases are caused by the successive action of primary stress factor(s) followed by the organisms of secondary action that can successfully attack only weakened trees.