Erwinia amylovora is the quarantine organism responsible for the fire blight of rcsacecus plants. The disease can affect pear tree, apple and loquat, and to diverse ornamental plants. This bacterium and the disease it causes have been studied in several projects that focused on setting up diagnosis systems, the characterization of Spanish strains and the study of some not very well-known aspects of the biology of this pathogen, to be able to optimize the strategies for the preventive control of the disease under Spanish conditions.The studies of variability of Spanish isolates have demonstrated the existence of strains with different plasmid content, and the research has focused on the study of the presence of a new plasmid involved in symptoms development and its characterization. Also a new species of erwinia pathogenic on pear, E. Piriflorinigrans, was discovered and its plasmid content compared to that of the plasm ids in E. amvtovcra (pEA29) and E. pyrifoliae (pEP36).