If the temperature is 65 F or above and relative humidity is 60 percent or more, or 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 may have droplets of ooze on them. The bacteria usually enter the flowers through natural openings such as stomates.
Wounds are also important entry points to leaves, shoots, and fruit.
Aphids, leafhoppers, lygus bugs, and other insects with piercing mouthparts may transfer fire blight bacteria directly into susceptible tissues. Wounds from hail often lead to a severe outbreak of fire blight. Any fresh wound can serve as an entry point. Disease management Temperatures just before and during bloom will determine if fire blight becomes serious in early spring. Daily temperatures must average 65 F or above during pink through petal fall for bacterial populations to grow enough to cause severe disease.