Plant diseases can be identified in the field or in a laboratory. In the field a grower or scientist uses visual identification. This is observation of the plant for symptoms such as spots, patches, discoloured or distorted leaves, stunted growth or a mosaic of light and dark green. The symptoms can be compared with descriptions of known diseases. A sample can also be looked at under a microscope to find the pathogen, for example fungal threads on a leaf. Observation and microscopy are quick, but many problems cause similar symptoms. For example, yellow leaves can be caused by a pathogen or by a mineral deficiency.
In the laboratory the pathogen itself is detected. DNA testing looks for DNA that belongs to the pathogen in a sample from the plant, because each type of pathogen has its own DNA. Another method detects an antigen, a molecule on the surface of the pathogen. Antibodies that bind to that antigen are used, and a positive result shows the pathogen is present. Laboratory tests can tell apart diseases with similar symptoms.