Posted: 08/07/2015 at 14:42
John, regardless of the water, the mix in the pot is good quality and there should be suffifcient calcium available to the plant. Has the plant been fed? If so, all decent tomato fertilisers contain calcium.
If your other plants are BER-free and in the same or similar mixes, and have been fed, the water isn't the problem.
BER is about the most frustrating tomato problem. I found one infected tom on one of my plants last night. The plant is one of 16 that enjoy absolutely identical growing conditions. One infected fruit out of about 100 toms across the plants, and the infected fruit is the single, solitary sufferer amongst about 10 toms on that plant.
Irregular watering is believed to be one of the causes of plant stress but there can be others - strong winds, fluctuating temperatures, extreme temperatures; anything that can upset a plant's equilibrium.
And, sometimes, as in my case, there seems to be no explanation.