Erratic water supply which impairs delivery of soil calcium,

See Tomato FAQ, 2nd item on the list
http://faq.gardenweb.com/faq/tomato/

Here is a link that might be useful: BER

jean001a:

Could be... but our garden is on soaker hoses on auto timers so watering is consistant. This year only one plant of the 24 had a bit of it & the epsom salts took care of it.

Some kinds are more likely to get BER than others.

Reasons for things to go wrong with your system:
- The system may not be running long enough and/or often enough.
- Even if the system is fine as to amount & frequency, a hot and/or windy and/or very low humidity can use up all the available moisture

Temporary shade may be needed at that time or perhaps run a 2nd cycle the same day.

slow lime mixed with soil at time of planting will prevent blossom end rot