Although blossom end rot looks like a disease, it is actually a 'physiological disorder' or general plant problem, caused by incorrect growing conditions.
Blossom end rot is a common problem of tomatoes, peppers and aubergines – especially those grown in a greenhouse – and is caused by erratic watering and/or excessive dryness at the roots. It is often a common problem of plants grown in growing-bags that are not wtaered properly.
Brown sunken spots develop at the bottom of the fruit opposite the stem.
Ensure the compost is kept evenly moist and not allowed to dry out. Giant Planters conatin twice as much compost as ordinary growing-bags, so are less likely to dry out between waterings.
Pick off affected fruit and change your watering technique to applying less water twice a day, rather than an occasional flood followed by drying out.
Feed every week with a soluble tomato food, since a lack of nutrients also adds to the problem.