Infection of blossoms is favored by temperatures of 76 F accompanied by precipitation or very high relative humidity. Infected blossoms wilt, turn brown, and die. Most of the leaf and twig infection is caused by bacteria blown through air in bacterial strands. Rain dissolves the gelatinous strands, freeing the bacteria for entry into natural openings in leaves and bark. The leaves that are killed by fire blight on apple turn brown, and the leaves of pear turn black, but don't fall off the tree.
Infected twigs become watery, dark and green and somewhat oily.
The infected blossoms, leaves, and twigs give the appearance of having been scorched by fire. Control Experts disagree as to the effectiveness of sprays on this disease, however Bordeaux or Mancozeb are claimed to be controls for fireblight. Should you decide to try, it should be applied first at pink stage 3 to 4 days before blossoms open and repeated every 5 to 7 days for about 3 weeks. Fire Blight cannot be controlled by spraying after this time. Losses from this disease can be reduced if home owners follow a blight prevention program of sanitation and spraying.