AbstractDieback of poplar and willow, grown in short‐rotation forestry plantations, was shown to be a result ofPseudomonas syringaeinfections in combination with freezing stress. Bacteria isolated from damaged poplar and willow trees at several sites in Sweden, identified asP. syringae, showed ice nucleationactive (INA) properties in laboratory tests. To confirm the effect of bacterial infection, five differentP. syringaeisolates were reinoculatcd into stems ofSalix viminalis, S. dasycladosandPopulusspp. in the greenhouse before application of sub‐zero temperature stress. Subsequent dieback of the shoots was observed. It is concluded thatP. syringaeis a Potentiall severe pathogen of willow and pop