Young succulent shoots are infected. Overhead sprinklers may aggravate the problem, especially in fire blight susceptible cultivars.
Damage ranges from individual flower or shoot infections to death of limbs.
Apple cultivars vary in susceptibility and extent of damage. For example, in Granny Smith, infections are usually limited and do not cause severe structural damage to the tree whereas Gala and Fuji trees may be devastated. The rootstocks M26 and M9 are very susceptible to this disease. This disease can be difficult to control. Make applications every 3 to 5 days until the end of bloom.