The hot sun may have dried out the flowers before they were pollinated, also late frost may have affected them and you need a pollinator to set the fruit. Welcome to GOY.

A similar question was asked a few days ago. Several things it could be. Firstly, a fruit tree that is growing in a pot or tub is open to being overwatered or underwatered and any stress on a fruit tree can cause it to drop its flowers or fruit. Early in the season, late frosts can be a problem. The other reason could be that the flowers were not pollinated, so another domestic apple tree or even a crab apple might help. It might not be all bad news as it is better for an immature tree not to fruit too early in its life so that its energy is used to build up a strong root and branch structure.

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