Primary Causes of Blossom Drop
Temperature: Tomato plants drop their flowers under extreme temperature regimes, such as high daytime temperatures (above 85°F), high nighttime temperatures (above 70°F), or low nighttime temperatures (below 55°F) (Table 1). Optimal growing conditions for tomatoes are daytime temperatures between 70°F and 85°F. While tomato plants can tolerate more extreme temperatures for short periods, several days or nights with temperatures outside the optimal range will cause the plant to abort flowers and fruit and focus on survival (Mills 1988). Temperatures over 104°F for only 4 hours can cause the flowers to abort. If nighttime temperatures fall below 55°F or rise above 70°F, or if daytime temperatures rise above 85°F, the pollen becomes tacky and nonviable, preventing pollination from occurring and causing the blossom to dry and drop (Chester 2004; Levy, Rabinowitch, and Kedar 1978; Mills 1988; Ozores-Hampton and McAvoy 2010).
Low temperature: Low temperatures interfere with the growth of pollen tubes, preventing normal fertilization. The pollen may even become sterile, which causes blossoms to drop.Tomato fruit do not set until nighttime temperatures are above 55°F for at least two consecutive nights (Chester 2004; Ozores-Hampton and McAvoy 2010).
High temperature: Sustained high temperatures, especially at night, rapidly deplete the food reserves that are produced in the tomato during the day. The result is sticky pollen, altered viability, and poor or no pollination. Ultimately, the blossom dries and falls off. Female flower parts can also undergo morphological changes, such as drying of the stigma (Mills 1988; Ozores-Hampton and McAvoy 2010).
If the temp is not right you're basically screwed.
http://edis.ifas.ufl.edu/hs1195
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