Bacterial contamination of blossoms occurs much more rapidly if there is a near-by active canker. Bacteria also tend to infest an increasing percentage of the flowers as the primary bloom period advances toward petal fall. The model user is asked to take in to account the recent history of blight in the area around the orchard, observe the stage and number of bloom, and set appropriate situation-specific degree hour thresholds. Never assume that E. amylovora is not present. As the blight bacteria can thrive only about four days on each individual flower, the bacteria must develop to dangerous numbers during the immediate three or four days leading up to blossom wetting.
Warmer temperatures allow rapid bacterial growth in flowers.
If bacteria numbers exceed a certain minimum while the flower is in good condition, then the flower is lightly wetted, infection is possible. Watch temperatures over time. Blossom wetting alone does not cause fire blight. Rain during cold or cool weather does not lead to infection, or blight would be common everywhere, every year. Moisture can be from rain, dew or light irrigation wetting.