Tomato plant wilt
Look closely at the leaves and see if there are any brown spots that appear before the leaf begins yellowing. If spots are evident, it would indicate a fungal disease. Remove all infected leaves and spray with a fungicide indicated for use on vegetables. Do not water overhead and repeat fungicide applications per instructions on the container.
If there are no spots evident then the pattern of lower leaves wilting and yellowing could indicate Fusarium Wilt which is considered to be a warm-weather disease. This disease is soil-borne and can survive in the soil for many years. See the web site below for more information and pictures of typical tomato diseases. Hope that helps.