Cause : If the yellow leaves appear at the base of a tomato plant, it can be a sign of aging and that the plant may be beginning to die. It can also be caused by a nitrogen deficiency in the soil. On occasion, other factors may cause yellow leaves including pest, fungus and bacteria infestations. These three causes usually produce other symptoms besides plain, uncurled, yellow leaves at the base of the plant. For example, if the leaves are turning yellow or brown higher up on the plant, it may be a sign of early blight. Have your soil tested at a local nursery to confirm the nitrogen-deficiency diagnosis.
Solution : If the soil have a nitrogen deficiency, supplementing the soil with well-rotted manure or compost, both of which are rich in nitrogen. You can also apply a nitrogen-rich fertilizer.