Spores attach themselves to the plant, germinate in a film of moisture within five to six hours and penetrate the leaf surface through stomata. Early season infection of fruit also occurs through stomata. Later in the season, infection of fruit occurs through cracks in the cuticle or via wounds and possibly lenticels. Often, harvest injuries may become infected and the fruit may decay during or after storage. Throughout the growing season, infections occur through wounds in the bark or on killed wood.
Cankers are a source of inoculum which can initiate leaf, fruit, and wood infections.
Old fire blight cankers, winter-injured wood, and dead prunings left in the tree often serve as sources of inoculum. Remove cankered wood from the orchard or mulch the brush so that it decays over the period of a year. Inspect trees for apple mummies and remove them from the orchard if possible, since mummies remaining in the trees from the previous season can also serve as a source of inoculum. Observe 25 fruit and leaves on each sample tree from mid-season through the preharvest period. Leaf spots may indicate an inoculum source for fruit rot infections.