Utah State University Extension
Fire blight is caused by a bacteria called Erwinia amylovora. Some varieties of apples are very susceptible to this disease, such as Jonathan, Jonagold, and Gala. Blossoms, terminal twigs, and sometimes entire limbs or trees are killed.
Murray, Marion and Alston, Diane, "The Backyard Orchard – Apple Pests" (2011). All Current Publications. Paper 968.