ABSTRACT
Disease diagnosis is the act or process of biotic or abiotic disease identification. Disease may be biotic or abiotic. Some plant disease agents cause visible symptoms distinct enough to allow for disease identification to be made relatively quickly on the basis of a visual study of only plant appearance. For example, crown gall caused by the bacterium Agrobacterium tumefaciens, induces irregular galls to form on the lower portions of stems and upper sections of roots of infected plants. These galls are often distinctive and allow immediate diagnosis. However, in many situations, visual appearance of the diseased plant is not plain enough to allow for an exact diagnosis of the problem. In these cases, diagnosis depends on one or more assays or tests in addition to a visual inspection of the plant. These added studies may involve soil pH testing, soil analysis (total soluble salt and fertility and nematode assays), light and electron microscopic study of the damaged plant tissues, cultural and physiological studies of the isolated pathogen, molecular studies of the pathogen [serology, gel electrophoresis, gas chromotography,
PCR (polymerase chain reaction), and DNA probe identification], plant tissue analysis, and soil or pathogenicity studies. The exact procedures used to diagnose a plant disease depend on the suspected disease and the plant or crop situation. The crop situation or the crop owner often dictates the level of specificity needed in the diagnosis. For a backyard garden, the identity of a corn (Zea mays) leaf spot as a Cercospora species leaf spot may be sufficient, but to a plant breeder concerned with the leaf spot damage in a field situation, the identity of the leaf spot needs to be more specific, and the leaf spot would be identified by genus and species, such as a Cercospora zeae-maydis leaf spot.
