The following spring, pustules will develop within these overwintered lesions. These pustules produce spores that perpetuate the spread of the disease. The fungus overwinters in leaves on the ground and also in infected twigs. Infection of pear foliage and fruit occurs under conditions similar to those required for infection of apple by the apple scab fungus. Ascospores are the major source of primary inoculum.
Infection occurs in the spring around bud swell.
Ascospores in the overwintered leaves are released as the result of rain and are carried by air currents to young leaves and fruit. Ascospores continue to mature over a 6 to 8 week period. Conidia are the source of secondary inoculum and are produced in twig lesions. Many secondary cycles may occur over a growing season. The length of the wetting period and temperature required for infection depend on the number of hours of continuous wetness and the temperature during this wetting period.