There are several possibilities why apple trees bloom but fail to set fruit:
- Apples are not self-fertile and need another apple tree or tree(s) of another different variety (or varieties) so the flowers can be pollinated (by bees, see #2.) and set fruit.
- Apple flowers rely on bees to carry the pollen from one variety to another so the flower(s) can be pollinated and the fruit can set and begin to grow. Both domestic honeybees and wild pollinators (bees, wasps) can do the job but you need.
- Bad weather -- including rain, wind, and frost/freeze can prevent bees from flying and pollinating the flowers. Temperatures that drop as low as 28 degrees F. during the pre-bloom, bloom, and post-boom period can damage the flower parts and prevent them from being pollinated. This is why it is best to situate orchards on a hill or out of 'frost pockets.'
- Lack of pest control -- many insects attack the set fruit shortly after the petals fall off. Serious infestations can keep the set fruit from growing. The little 'fruitlets' will fall off and there will be no fruit to grow and mature. Don't spray insecticides during bloom to protect the bees, but as soon as the flower petals fall off, spray with an insecticide to prevent crop apple damage and/or complete loss of the crop.