As fruit trees break dormancy and buds begin to grow, they become more susceptible to damage by freezing. Fruit buds pass through up to eight stage from being fully dormant to being in full bloom. Damage depends on the duration/intensity of the freeze, the stage the buds are in, and somewhat on the cultivar being grown. The position on the tree can make a difference too. Fruit buds positioned lower in the tree may freeze while those higher in the tree escape damage. Research done by H.J. Larsen, Colorado State University showed that for apple trees at silver tip, 15 degrees F for 30 minutes resulted in 10% kill of buds, and 2 degrees F for 30 minutes  resulted in 90% kill. Once apples get to first pink, the corresponding temperatures are 27 degrees F and 21 degrees F, and at full bloom, 28 degrees F and 25 degrees F.  For peach fruit buds, a temperature of 18 degrees F for 30 minutes resulted in 10% kill of fruit buds that were dormant, and a temperature of 1 degree f for 30 minutes resulted in 90% kill. At first swell , the temperatures for the same level of bud kill are 21 degrees F and 5 degrees F, and at full bloom, 28 degrees F and 25 degrees F. keep in mind that these are general guidelines, and the site conditions will also be a factor.