You should contact your local Extension office and ask about their plant diagnostic services. After examining the affected fruit, they may be able to give you an exact answer. The following information addresses one possible cause.
Pear rust mites—Epitrimerus pyri
Pear rust mites feed on the surface of fruit and foliage, causing a bronzing of the tissue. Injury to leaves may stunt the growth of young trees. After petal fall, populations may develop on fruit around the calyx or stem end, giving a localized russetting to those areas. Russetting may spread over the entire fruit surface.
Solutions
In backyard situations, natural enemies usually keep pear rust mites under fairly good control. Damage does not generally justify treatment in backyard trees. If treatment is necessary, best control is achieved with a fall application of sulfur or lime sulfur and oil in November.
