The disease also can occur on hawthorn, mountain ash, serviceberry, pyracantha, cotoneaster, blackberry and raspberry. Disease Cycle Fire blight is caused by the bacterium Erwinia amylovora . In spring, when temperatures reach 65 degrees F and frequent rain occurs, bacteria resume activity and multiply rapidly. Masses of bacteria are forced through cracks and bark pores to the bark surface, where they form a sweet, gummy exudate called bacterial ooze. Insects such as bees, ants, flies, aphids and beetles are attracted to this ooze, pick up the bacteria on their bodies, and inadvertently carry the bacteria to opening blossoms.
Bacterial ooze splashed by rain also can spread the pathogen.
Once in the blossom, bacteria multiply rapidly in the nectar and eventually enter the flower tissue. From the flower, the bacteria move into the branch. All flowers, leaves and fruit above the point of infection die. Droplets of ooze can form on these infected twigs within three days. Fruit may be infected through insect wounds.