In all plants presenting blister bark symptoms Pseudomonas syringae pv. syringae van Hall was isolated.
Plants characterized by the poorest physiological status were attacked in the presence of a low inoculum.
Vigorous plants were infected in the presence of high environmental inoculum.
Symptoms occurred earlier in autumn on the stressed plants, and after winter frost in the vigorous plants.
Plant vigour was related to soil type and consequent root behaviour.