YEP...All Day Long, Mike...thats BER.

Is BER caused by the plants getting wilted and then being watered? Could a lack of calcium also be a factor? Container tomatoes seem to need more supplementation that in-ground tomatoes.

A Tomato plant is Very Sensitive, when bearing fruit, to soil moisture fluctuation. It expects no such drastic fluctuation, and when grown in good tilthy earth, it will respond naturally, with it's roots, to resolve the issue to it's own content. Growin in a pot....it dont have that option...)))

During a major heat-wave last week, the plants became very limp and wilted, despite daily watering

I guess I'm the odd person out here, LOL, but to me it doesn't look like BER.

The conditions you describe could be allowing for any number of normal environmental bacteria and fungi to get into any breaks in the skin when conditions are damp and hot and there's little air movementand initiate a rotting process. Black Mold Rot can appear exactly as your picture shows.

And it could be BER as well, it really could, but you'll know in the next few weeks b'c if it is BER it will go away.

Carolyn