In woody tissue there may be discolouration of the internal tissue which when cut open are brown to red in colour. The blossoms of affected trees will be water-soaked and discoloured and will also quite often be covered in bacterial ooze. Fruit may be affected and they stay a small size and have a shrivelled appearance and also have bacterial ooze present. The bacterium survives in winter in bark along the edges of cankers from infections the previous year. In spring it multiplies and is then spread by rain and insects, especially to new blossoms.
Bees are particularly active in spreading the bacteria.
After penetration it kills cells and spreads through the plant. A susceptible plant, a virulent pathogen and favourable weather conditions are required for extensive disease development. Moist wet weather with temperatures above 15oC are required for high disease levels. Sprinkler irrigation and high levels of fertilisers can favour the disease. The disease is worse on apples and pears but can also be found on other related plants such as Cotoneaster, Sorbus, Pyracantha, Photinia, Crataegus and Chaenomeles.