Answer: You'll probably find that most references to calcium deficiency in tomatoes refer to "blossom end rot". Rather than being caused by a disease, this symptom is caused by poor water relations. For example, if a dry spell is followed by a heavy rain, blossom end rot is a likely result. Gardeners try to keep soil moisture as even as possible--a thick layer of mulch sometimes does the trick.
The actual mechanism for blossom end rot involves calcium--check out this web site for more information and details of how calcium affects water relations in tomatoes.
I hope this is helpful.
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