West Sussex,
United Kingdom 
My tomatoes are rotting from the bottom up on the vine as they ripen, what is it and how do I stop it
31 Jul, 2010
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That's probably what's called "blossom end rot". Not actually a disease, but a calcium deficiency in the fruit, usually caused by insufficient soil moisture at night. If they are growing in containers, I would water more often, and maybe add lime or gypsum to the soil. If they are growing in the ground, water more deeply, and put about 5 cm of coarse mulch around the plants, at least as far as the foliage spreads. Lime may be useful in the ground, too, if the soil is very acid.
31 Jul, 2010