Flower bud necrosis of commercial pear trees during winter dormancy is the most destructive phase of bacterial blast of pear caused by Pseudomonas syringae in the fruit tree-growing area of Catalunya, Spain. Forty percent of strains of P. syringae isolated from buds were ice nucleation active (INA) and 70% were phytopathogenic to pear. Thirty-nine percent were both INA and phytopathogenic. They were capable of developing blast of dormant buds in the absence of cold depending on the inoculum dose. However, cold temperatures increased disease incidence in inoculated flower buds from cut branches forced to bloom (...)