tomatoes are rotten underneath

If you've got brown mottled colour sounds like blight is it on the leaves and stem??

Do you mean that the tomatoes themselves are going brown and rotten at their bases? If so - one reason could be "Blossom end rot"
This happens if the watering has not been very even and the compost dries out - something tomatoes hate. Those fruits are of course useless - just bin them but as long as you sort out the watering asap ..the rest of the crop should be OK. Tomato bags are very difficult to keep watered ..if that is how you are growing these tall plants.

hi yes they are in grow bags but in pots with the bottom cut out first, it probably is the watering i will take extra care thanks.it is only on the fruit