Bees do not feed on bacterial ooze but can visit infected flowers and spread bacteria to uninfected flowers. Flies and ants are attracted to the bacterial ooze and can then carry it to uninfected blossoms. Aphids, flies, ants, leafhoppers and tarnished plant bugs have been implicated in the spread of fire blight. Primary insect vectors include ants, flies, and wasps. Symptomless pears are not a source of the pathogen, which is rarely detected (1 out of 5,600 fruit) as an epiphyte on commercially-produced fruit. Entrance into healthy appearing leaves and shoots may be through the base of epidermal hairs. Vigorously growing shoot tips, young leaves, and wounds also can be infection sites. Flowers, which are open for up to 3 days, support rapid growth of the bacteria while older flowers do not. A temperature of 65☏ or higher in a 24-hour period plus a trace of rain or high humidity (greater than 65%) is necessary for infection.

Bacteria enter healthy, main bloom or rattail blossoms through the stigma, nectary, anthers, and sepals. Insects, pruning tools, and splashing rains spread this bacterium. Risk of infection increases with the number of more active 'holdover' cankers in an orchard. Cause Erwinia amylovora, a bacterium that overwinters in cankers on infected pear, apple, and some ornamental trees.