The bacterium is native to North America and was accidentally introduced into the UK in 1957. It was formerly a notifiable disease but this is no longer the case in Great Britain; however it is not yet established on the Isle of Man or the Channel Islands. Suspected cases in these areas should be reported to the relevant plant health authority.
The bacteria overwinter in bark cankers. In warm, wet and windy weather in spring, bacteria ooze out of the cankers. Infections occur when the bacterium gains entry to the inner bark, usually via the blossoms, and it is spread by wind-blown rain and also by insects including bees.
Under favourable conditions the infections spread rapidly down the inner bark at up to 5cm (2in) per day, staining the cambium a foxy reddish-brown colour. Severely attacked trees appear to have been scorched by fire. Most years in the UK are too cold at blossom time for infections to occur and the disease is usually of relatively minor importance. A particular risk of infection occurs when trees produce a secondary, small flush of blossom later in the season when conditions are warmer.