I have several Roma tomato plants. One of them in particular started yellowing quite a while ago. It all started from the bottom leaves and gradually up. The top leaves are so far so good and green. Overall, 4/5 of the plant is yellowing, yet it still produces fruits. It has kept yellowing more and more. What's happening to it? I also noticed that one of other plant just began to start yellowing too. The pictures I took are all from the severe one, except the one with the pest. Is this the guy who has made the plant yellowing? However, it is on a healthy plant. Thanks.