A serious disease of pears and other trees of the family Rosaceae caused by the bacterium Erwinia amylovora. The disease attacks blossoms and leaves in the spring and, as the disease progresses, individual branches or whole trees look as if they have been scorched by fire. The host forms *cankers
in response to infection. The bacteria overwinter in the cankers and are spread by insects and rainsplash to infect flowers in the spring. Hawthorn (Crataegus), an *alternate host
, can also be a source of inoculum.