Fire blight infection occurs when blight bacteria are carried to the flower stigma tip, grow for three or four days, then are gently washed into the flowers' nectaries.
Infection does not occur unless the bacteria are in high numbers. The development of high numbers of bacteria on the stigma tip requires warm daily temperatures, usually in a range between 78 and 90°F, but infection may occur during slightly cooler conditions if there is a recent history of blight infections in the orchard or orchard neighborhood. Blight is much more likely if there was fire blight infection in the area the previous year.
Flower wetting is a critical aspect of infection. Wetting that triggered flower infection has occurred from rain, dew of about two hours or longer, misting from nearby irrigation, and light wetting from any form of sprinkler irrigation. Wetting from sprayers has not apparently triggered blight in the Pacific Northwest.
While the DAS makes every effort to assist you to determine that wetting has occurred on the weather monitoring site, you must be aware that the temperature and wetting measurements are taken in non-irrigated sites outside of the more humid orchard, and undocumented wetting may have occurred in the orchard, especially in frost pockets, draws or similar areas with poor air drainage leading to higher humidity and dew.
(Tim Smith, WSU Extension)