Posted: 17/07/2015 at 08:08
pansyface is right, plum varieties are more prone to BER than others. No one knows why. I've had them in the ground alongside other varieties, identical growing conditions and watering, the plums can be riddled with BER while the other varieties are clean. Very frustrating if you like growing plums.
That aside, BER is caused by plant stress, the stress interfering with the plant's internal mechanisms that deliver calcium to the fruit. Years ago it was believed to be insufficient calcium in the soil but science eventually put that to rest. Yet you still see and hear the myth perpetuated.
Plant stress can be caused by any number of things, irregular watering only one of them. Sometimes it's impossible to identify the cause. I have a beefsteak variety in the ground, one of 16 plants growing side-by-side in identical conditions. The plant has tossed up half a dozen fruit with BER, the other 15 plants none. I've grown the variety before without a hint of BER. Go figure.