The taxonomy of plant pathogenic bacteria, with its three interrelated aspects of classification, nomenclature and identification, is a central aspect of bacterial plant pathology. It is clearly important to be able to establish the identity of an isolated plant pathogenic bacterium, so that the agent causing a particular disease — or with the potential to cause disease — can be clearly defined. Phytopathogenic bacteria may be isolated from a wide range of sources, including infected plant tissue, seed surfaces, soil and water environments, and the identification of bacteria from such sites has implications not only for disease diagnosis and pathogenicity but also for studies on disease epidemiology and aetiology. In addition to defining bacteria as agents of disease, taxonomic studies can also provide useful insights into phylogenetic relationships between the phytopathogens.

The taxonomy of plant pathogenic bacteria will be considered in relation to two major aspects:

1. The establishment of a clearly defined and internationally accepted system of classification and nomenclature.

2. Bacterial identification, including: isolation and identification from different sites, pathogenicity testing, in vitro diagnosis, and computer identification by numerical analysis. There is now a wide range of features on which bacterial classification can be based, and which can be used for identification.

General principles of bacterial taxonomy, with details of the various characteristics that can be used for classification, are discussed in Bergey's Manual of Systematic Bacteriology (Kreig & Holt, 1984).