Blossom End Rot: If your ripening fruit develop a dark spot at the lower (blossom) end, a spot that gradually widens and deepens, you’re looking at blossom-end rot. It’s an environmental problem most often caused by uneven watering or by calcium deficiency. (These can be related; uneven watering can interfere with the uptake of calcium.) The simplest treatment is therefore pre-treatment: make sure soil is rich in all necessary nutrients (including calcium), and water regularly. Mulches also help maintain even moisture levels (see Mulches for the Home Vegetable Garden).
Subject: Tomato question