My Better Bush tomatoes don't ripen. They get mottled green/red and then rot from the inside out. Now some are starting to rot on the vine while they're still green. There's no visible rot at the blossom end and no brown on the skin.

The plant is growing in a bag of garden soil. I fertilize it with diluted fish emulsion when I water. The temperature here is around 90 degrees at midday.

Can anyone tell me what's wrong and how to fix it?