Posted: 23/07/2014 at 13:30
paull, plum varieties (lincluding San Marzano) are simply more prone to BER than other varieties. No one knows why. Must be something in their genes.
Erratic watering isn't the only cause of BER. BER occurs as a result of plant stress that creates problems with the plant's internal systems. Anything that stresses a plant can feasibly cause BER. It could be excessive humidity. It's really a matter of process of elimination. Trouble is, you don't get instant answers. And sometimes there's no obvious answer.
BER is one of the curses of tom growing.