Blossom­ drop is a condition suffered by tomatoes, peppers, snap beans, and some other fruiting vegetables in which the plant blooms but fails to set fruit, and then the blooms die and fall off. Tomato plants lose their blossoms for several different reasons, usually related to some kind of stress. The stress may be nutritional, environmental, or some combination of the two. Extreme temperatures and changes in humidity levels may cause flowers to drop. Anything which interferes with the pollination and ­fertilization process may result in flower drop. Without pollination, which stimulates fruit set, the flowers wither and die.

Tomato blossoms are self-fertile, and a certain amount of self-pollination of tomato flowers does occur. But pollen does not move well by itself from anther to stigma, as evidenced  by the really poor pollination seen in greenhouses when no pollination aid is given.

Shaking by wind or mechanical means can help release the pollen, which drops down (the blossoms normally hang downward) through the stamen tube to the stigma. The best possible motion to release this pollen is provided by a bee that "sonicates." Sonication is the vibration of the wing muscles without flight, causing the whole flower to vibrate, and a cloud of pollen to be released onto the bee's body and, at the same time, onto the stigma.

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- Courtesy of NYBG Plant Information Service