Farmers and scientists need to find plant disease early and work out what is causing it. In the field, the first step is to observe visible symptoms, such as spots on leaves, yellowing, wilting, stunted growth, rotting or abnormal growths. Yellow leaves alone do not prove a pathogen, because a shortage of minerals, frost, drought or too little light can cause similar damage. So the possible environmental causes must be checked and eliminated, for example by testing the soil and looking at the weather.
Next comes distribution analysis: recording where the affected plants are. If damage follows a feature such as a waterlogged strip, a frost pocket or the edge of a field, the cause is likely environmental. If it starts at one plant and spreads outwards to its neighbours over weeks, the cause is more likely an infectious pathogen.
To identify the pathogen itself, a sample is usually taken to a lab. The pathogen can be cultured on agar and examined under a microscope. DNA analysis can match it to a known species. Test kits that use antibodies, which bind to antigens on a particular pathogen, can give a result quickly, and some can be used in the field.