Tomato diseases are plentiful and complex. There is no substitution for contacting a local extension agent for help in a final diagnosis. There are many websites, such as the Cornell site, that provide a wealth of information.
Yellow leaves can indicate a number of conditions. Lower foliage with yellow blemishes is likely indicative of a fungal issue such as early blight. If an entire tomato stem or plant turns bright yellow and wilts, a more serious issue is underway, such as fusarium wilt. Fusarium attacks plants through the roots and resides in the soil for years. A general “yellowing” of foliage (pale rather than deep green) could indicate the need for added nutrition or the available nutrients being out of balance.