Utah State University Extension
Fire blight is caused by a bacterium called Erwinia amylovora. Pears are very susceptible to fire blight. Blossoms, terminal twigs, and sometimes entire limbs or trees are killed. Environmental conditions favoring infection are warm, rainy days during bloom.
Murray, Marion and Alston, Diane, "The Backyard Orchard – Pear Pests" (2011). All Current Publications. Paper 972.