Guidelines to Identification and Management of Plant Disease Problems: Part I. Eliminating Insect Damage and Abiotic Disorders
http://edis.ifas.ufl.edu/mg441
Beginning master gardeners can be intimidated by the daunting task of correctly diagnosing the cause of a client's plant health problem. However, with experience and the development of a systematic approach to diagnosis, master gardeners will become confident and successful plant problem detectives.
Plant health problems result from 1) plant pathogens causing diseases; 2) insects damaging the plant; or 3) environmental factors, often referred to as abiotic disorders. Unfortunately, the symptoms for these health problems are often very similar. A good approach to identification of plant disease is to begin by deductively eliminating insect damage and abiotic disorders as possible alternative causes.