Symptom(s):

The blossom end (opposite the stem) of the tomato develops a black spot.  This can occur at any stage of development.  This is actually not a disease but is a failure of the tomato cells to develop properly.

Cause(s):

One of the causes of tomato blossom-end-rot is the failure to get calcium to the developing fruit.  This is quite often due to lack of calcium in the soil and/or the ability of the plant to get the calcium to the fruit due to lack of water.  The tomato can still be eaten.  Make sure that the plant is watered on a regular basis.

Control(s):

Amend the soil by adding calcium in the form of a purchased chemical or using crushed oyster shells and/or crushed egg shells in the hole when you transplant the tomato.

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Reference(s):