The combination of wet soil/compost and cold temperatures overnight, will give tomato plants a hard time.
Here are a few tips:
- Water in the morning so plants can use most of the water they are sitting in before the temperature drops at night.
- Don't sit tomato plants that are in pots, in a tray of water for more than 15 minutes or so, otherwise the soil will become sodden.
- Roots need air as well as water, so a root ball that is constantly wet will not grow to its full potential.
- Tomato plants do not like wet leaves for more than a short time ... wet leaves overnight can trigger tomato blight and other fungal disease.
- Keep plants indoors or under cover at night until all danger of frost has past. In the UK that will be around the end of May.
New growth should show a good leaf colour and eventually the lower leaves can be removed. Most of the plants energy goes into the new growth and the old leaves get left behind - as it were.