Bacteria overwinter in the margins of cankers on branches and trunks. When the temperature reaches about 65°F, bacteria begin to multiply and appear on the outsides of the cankers in drops of clear to amber-colored ooze. The bacteria are spread to blossoms primarily by rain, with some transmission by flies and ants. Blossom-to-blossom transmission is carried out mainly by bees and other insects that visit the flowers. The spread of the bacteria from flower to flower by bees is rapid. Insects also transmit bacteria to growing shoots. If the temperature is 65°F or higher and relative humidity is at 60 percent or more, or if there is rain, new infections can occur. At 75°F, blossom blight and shoot blight will be evident in 4 to 5 days. Bacterial ooze appears on the new infections soon after the symptoms, providing additional sources of bacteria for new infections. In early to midsummer, during prolonged periods of muggy weather, blighted shoots and spurs, infected fruit, and new branch cankers all might have droplets of ooze on them.
The bacteria usually enter the flowers through natural openings such as stomates. Wounds also are important entry points to leaves, shoots, and fruit. Aphids, leafhoppers, lygus bugs, and other insects with piercing mouth parts can transfer fire blight bacteria directly into susceptible tissues. Wounds from hail and wind-driven rain often lead to a severe outbreak of fire blight. Any fresh wound can serve as an entry point.