We all want to get an early start on our tomatoes, so they are ready to ripen by July; however, we are at the mercy of the weather. Too much moisture or June gloom can bring on the spotting and yellow leaves. Bacterial blights and leaf spot like septoria or alternaria can be controlled by carefully pinching lower leaves that appear infected. Try to avoid backsplash from the soil when watering. Serenade (bacillus subtilis) is a promising organic control.
Viral diseases that begin with yellow leaves then progressive wilting need to be destroyed. Fusarium and verticillium wilt are caused by damage to the structures inside the stems where the plant is unable to take up water and nutrients. These viruses live in the soil for years. Crop rotation or pots are your best solutions in these cases. I routinely grow some in raised beds and some in pots, staggering my plantings to include very heat tolerant varieties for mid-season fruit-set.