The old adage ‘You can’t fix a problem if you don’t know you have one’ underpins the basic science of diagnosing plant diseases and nutrient deficiencies. For years, farmers and scientists have worked together to identify a set of visual clues that can be used to determine diseases and nutrient deficiencies in a variety of agronomic crops. These clues and symptoms can be extremely useful, especially when soil and plant tissue testing methods are neither feasible nor available.