I have tomato plants that the bottom leaves turned yellow
Also, the small tomatoes are turning rotten on the vine.
Certified GKH Gardening Expert
There are a number of reasons for leaf yellowing in tomatoes. This article should help with pinpointing and correcting the cause of yours: https://www.gardeningknowhow.com/edible/vegetables/tomato/yellow-tomato-leaves.htm
The rotting is called blossom end rot. This article will help you with that: