Yellowing leaves on tomato plants are the plant's way of telling you something is off. Unlike the natural browning and drop of older bottom leaves, widespread or rapidly spreading discoloration is a warning sign worth taking seriously.
To thrive, tomatoes demand a fairly specific environment: steady moisture without sitting in soggy soil, nutrient-dense earth, plenty of direct sun, and a fighting chance against pests. Tip any one of those factors out of balance, and the leaves are usually the first to show it.
