Fire blight is a serious disease causing considerable damage and economic losses in apple and pear. The disease is caused by a bacterium, Erwinia amylovora (Burrill), which infects hosts in the Rosaeceae family. The host plants include quince, crab apple, hawthorn, cotoneaster, mountain ash and firethorn. Raspberry can be infected by fire blight bacteria, but the raspberry strain cannot infect apple, pear or ornamental plants. The bacteria E. amylovora is found in most areas of the province where apples are grown. The pathogen has caused most damage in southwestern Ontario, where, in some years, whole orchard blocks have been destroyed due to severe infections. Fire blight has become a serious problem in high density apple plantings that contain new susceptible varieties and dwarfing rootstocks. 

Fire blight attacks many different parts of the tree, and fire blight symptoms are often referred to by the part of the tree attacked – blossom, shoot, fruit, limb and trunk, and collar or rootstock blight.