The bacteria enter the host through natural openings in blossoms and leaves or through wounds in the bark. Wounding by insects, hail, pruning or wind whip is necessary for infection of twigs. Spread downward within the tree or shrub also occurs by rain splash. Infections continue to develop until the spring flush of growth stops or until about a month after flowering. Damage Accurate estimates of losses from fire blight are difficult to obtain, but no doubt range in the millions of dollars annually.
Fire blight is considered the most damaging disease of pome fruit in North America.
Unlike many other plant diseases, fire blight is destructive to the current year's crop, and will cause permanent damage to an orchard. It is extremely dangerous to the pear or apple industry in a fruit growing region. In the 1970s fire blight became prevalent on certain woody ornamentals, especially cotoneaster. The loss of an established crabapple tree or cotoneaster hedge in the home landscape can be significant in terms of aesthetic value, time, labor and replacement costs. Unfortunately, no single measure provides good control of fire blight.