Fire blight, caused by the bacterium Erwinia amylovora, is a common and frequently destructive disease of pome fruit trees and related plants. Pear (Pyrus species) and quince (Cydonia) are extremely susceptible. Apple, crabapple (Malus species), and firethorns (Pyracantha species) also are frequently damaged. Fire blight is less common on hawthorn (Crataegus species), Spiraea, Cotoneaster, toyon (Photinia species), juneberry or serviceberry (Amelanchier species), loquat (Eriobotria), mountain ash (Sorbus species), and other related plants. The disease can destroy limbs and even entire shrubs or trees.

In spring, branch and trunk canker symptoms can appear as soon as trees begin active growth. The first sign is a watery, light tan bacterial ooze that exudes from cankers (small to large areas of dead bark that the pathogen killed during previous seasons) on branches, twigs, or trunks. The ooze turns dark after exposure to air, leaving streaks on branches or trunks. However, most cankers are small and inconspicuous; thus infections might not be noticed until later in spring when flowers, shoots, and/or young fruit shrivel and blacken. The amount of fruit loss depends upon the extent and severity of the disease.