What causes a Tomato plants leaves to turn yellow?

It could be many things but I would guess you water them in the sunlight.

What causes a Tomato plants leaves to turn yellow?
Often plants turn yellow if they get too much water. But there are many different reasons from lack of sunlight to illness. I would start with checking for bugs and then checking that you are not overwatering. They like a good watering but not such that the soil does not dry out over the course of the day. If you find Bugs a good way to deal with them is to use a kitchen soap such as joy and water and put it in a spray bottle and spray down bugs such a aphids. I try to stay away from bug killers when it comes to food plants but soap usually works. If all else fails it may just be that they are not getting enough sun light.
Reply:overwatering


underwatering


insects


disease


age (older leves die)





hope this helps. :)
Reply:Over watering.
Reply:It could be red spider mites .They love tomato plants. They are a sucking insect that will turn the leaf yellow by sucking the juice out of the leaf. Kelthane is the best thing to use on them. Spray the underneath side of the leaf. Read the label to make sure you don't spray right before harvesting tomatoes.