Source: Mary Ann Hansen, Virginia Polytechnic Institute and State University, Bugwood.org
Blossom end rot of apple is not a major problem in Pennsylvania orchards. Because it occurs only infrequently, very little is known about its cycle and control.
Blossom end rot appears to be more common in seasons of prolonged cool, wet weather during and shortly after bloom. It has appeared most frequently on Delicious, Rome Beauty, and McIntosh. On stored fruit, especially Delicious, blossom end rot often leads to moldy core. Captan and Topsin M have shown efficacy; however, the efficacy of other fungicides is unknown. This disease is typically sporadic and losses are minimal.