How do you prevent blue mold on apples on the tree?

Tow weeks prior to budding spray dormant oil on the tree mixed with a lime/sulfur compound. Kits are available from most green houses and hardware stores. Spay treat the trees again after the blossoms have dropped. and do a repeat spray of lime/sulfur

when the Apples have set. A final spray in fall after harvest will help keep fungus and insects from overwintering.