During wet periods, spores ooze out of fruiting structures and are dispersed by rainfall. Infection of wood can occur through lenticels and wounds. It is not known exactly when fruit infection occurs. Some theories propose that infection can occur anytime from the bloom period to harvest, while other theories state that fruit infection only occurs during the last six to eight weeks of the growing season, and that the degree of infection is dependent on the sugar content of fruit of individual cultivars. There are also differences in cultivar susceptibility to white rot infection.
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. During early midseason, observe 25 fruit on each sample tree. Although there are some differences in fruit susceptibility among the varieties, all are susceptible.