The science which deals with cause of the plant diseases and nature of causal agent

The sciences that deal with the cause of plant diseases and the nature of causal agents are Etiology or Aetiology.
Etiology is the study of causation or origination. It is the study of the causes, origins, or reasons behind the way that things are, or the way they function, or it can refer to the causes themselves.
The word is derived from the Greek (aitiologia) "giving a reason for" (aitia, "cause"); and (-logia).