Bacterial diseases cause serious problems in cultivation of onion (Allium cepa L.) in Poland and abroad. There are several reasons of the losses in onion production caused by these diseases, e.g. weather conditions, biology and epidemiology of bacteria, lack of efficient pesticides, lack of disease-resistant cultivars. According to reports, onion is infected by following bacterial pathogens: Burkholderia gladioli pv. alliicola, Burkholderia cepacia, Pectobacterium carotovorum subsp. carotovorum, Dickeya chrysanthemi, Serratia plymuthica, Serratia marcescens, Enterobacter cloacae, Pantoea ananatis, Pseudomonas marginalis pv. marginalis and Pseudomonas viridiflava.