Dieback of blossoms and leaves on branches looking as though they have been scorched by fire (Hence the name Fireblight). The dead material has a thick black exudate that is full of the bacteria. Affects plants almost exclusively in the Rosaceae family.
Will affect Summer blossoming plants a lot more than early spring blossom. Most infectious during hot, wet weather. Over-fertilization and over watering can increase the risk of infection.
The bacteria infect the plant through stomata and start killing the leaves. They multiply and produce a thick black exudate which sticks to vectors or washes off in rain to spread the bacteria. Bacteria replicate through binary fission which means the number of bacteria doubles as fast as every 20 minutes.