Erwinia chrysanthemi mutants, containing transcriptional fusion of one of the pectate lyase genes (pelA, pelB, pelC, pelD, pelE, pelI, pelL, pelZ) with the reporter gene encoding b-glucuronidase activity, were studied for their ability to cause disease symptoms and to synthesise pectinases after inoculation of potato tubers. The strains affected in pelI and pelL genes displayed a reduced virulence on potato tubers demonstrating the important role of these isoenzymes in soft rot disease. Analysis of the bacterial population showed an active multiplication of bacteria during the infection. Similar kinetics of growth were observed for all mutants and for the wild type strain. Comparison of the mutants and the wild type strain showed that the pelI, pelL and pelZ mutants synthesised reduced levels of Pels. The expression of pelA, pelE and pelZ is 5-fold higher in planta than in in vitro. In contrast, both pelI and pelL are highly (10-fold factor) induced in planta, which is characteristic of the plant-inducible pectate lyases.