It is best to fertilize fruit trees in early spring, before the first budding. Fertilizer encourages new growth, which in turn leads to more fruit.
Fertilizing trees later in the season, especially in fall, can damage the tree, because new growth is not hearty enough to survive winter frosts and freezes. Additionally, too much fertilizer can encourage new leaves and shoots but not enough buds for fruit to form. Pruning can also encourage new growth on a tree without requiring fertilizer. If a tree has been pruned back more than normal, then fertilizer is not necessary, as the tree replaces the limbs lost.