Probable causes of Blossom drop [See Tamara's comment about temperature]
Night temperatures too low (below 55*F)
Day temperatures too high (above 90*F)
Smog during blossoming period
Excess Nitrogen fertilizer
Shaded by trees or buildings (not getting enough light)
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Keep soil moderately moist
Tapping on blossom stems 3x a week in midday when
flowers are open may help set fruit
Early blossoms do not consistently set fruit
source: U.C. Davis/Mary Louise Flint