Frost during bloom
A common problem is a late frost that destroys the delicate outer structures of the flower. It is best to plant apples on a slope for air drainage, but not on a south facing slope (in the northern hemisphere) as this will encourage early blooming and increase susceptibilty to frost. If the frost is not too severe, the tree can be wetted with water spray before the morning sun hits the blossoms, and it may save them. Frost damage can be evaluated 24 hours after the frost. If the pistil has turned black, the blossom is ruined and will not produce fruit.
Growing apples near a body of water gives an advantage by slowing spring warm up, which retards bloom until frost is less likely. Areas of the USA, such as the eastern shore of Lake Michigan, the southern shore of Lake Ontario, and around some smaller lakes, where this cooling effect of water, combined with good, well-drained soils, has made apple growing concentrations possible in these areas.
Home growers may not have a body of water to help, but can utilize north slopes or other geographical features to retard spring bloom. Apples (or any fruit) planted on a south facing slope in the US, will bloom early and be particularly vulnerable to spring frost.