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 leaves through natural openings such as stomates. Wounds are also important entry points to leaves, shoots, and fruit. Insects may 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.
Disease Management Temperatures just before and during bloom will determine if fire blight becomes serious in early spring. Daily temperatures must average 65 degrees F or above during pink through petal fall for bacterial populations to grow enough to cause severe disease. The disease also occurs later in the season when bacteria enter late-opening blossoms or growing tips of new shoots. Cut apple limbs at least 8 inches below external evidence of the canker and cut pear limbs at least 12 inches below. Where the disease was severe the previous year, apply a dilute Bordeaux spray plus miscible superior oil at silver tip.