Hi- fusarium wilt of tomato is a wilt disease. It does cause leaves to turn yellow but you would also see healthy lower green leaves (often on one side of the plant) go limp during the day due to the pathogen clogging up the water transport system.
If your plant is wilting you can slice through the very outer "skin" and see if you see vascular tissue that is darkened. This is a typical symptom.
If your plant is not wilting we suspect the cause of the yellowing and sub-par growth may be due to a combination of site issues. Some possibilities include inadequate sunlight, close proximity of plants to a heat absorbing/reflecting wall, incorrect soil pH and inadequate nutrients. Test your soil this summer or fall and follow the recommendations that come back from the soil testing lab: