Shoot blight develops from secondary infections that originate on young terminal shoots. Shoot blight usually develops in late spring or early summer. Trauma blight is a term used to describe infections that occur when blight is initiated at leaf or bark injuries resulting from hail or severe windstorms. Rootstock blight occurs when bacteria from infected blossoms or shoots moves internally through symptomless trunks and infects roots. E. amylovora overwinters in cankers on infected limbs.
Cankers become active in early spring as temperatures warm and buds begin to develop.
Active cankers produce a yellowish to white bacterial ooze that can appear several weeks prior to bloom. During bloom, pollinating insects rapidly move the pathogen from flower to flower initiating the blossom blight phase of the disease. Flower receptacles and young fruits are resistant after petal fall. Inoculum produced from infected blossoms is further spread by wind, rain, and insects. In years when blossom infections do not occur, the primary sources of inoculum for the shoot blight phase are the overwintering cankers.