Fusarium wilt is caused by Fusarium oxysporum f. sp. lycopersici. The first symptoms are a yellowing and drooping of lower leaves on a single stem. Early symptoms may appear on only one side of the plant. A progressive yellowing and wilting of the leaves occurs, and the plants may die. When the stem is cut open, the vascular (water conducting) tissues under the surface are frequently discolored brown. This vascular browning extends from the roots to the upper portions of the plant and into the leaf petioles. This fungus infects only tomato.

There are, IIRC, three different races of Fusarium. Many hybrids are resistant to one or more of them.

If you find vascular browning, you might ask your County Extension Service if they know which types of Fusarium are found in your area. Then next year you could select varieties which have resistance to those particular Fusarium races. (You'll see "F1," "F2," or "F3" in the description.)

Ohio county extension offices
(Scroll down a bit to find the county offices.)

Are the fruits affected?

It would help to see the entire plant.

Yellowing lower leaves can also result from low fertilizer and/or a water shortage. Much more common than fusarium wilt.

The tops of the plants look healthy. Lots of new growth. All the lower stems/leaves are in some stage of yellowing as seen above. The fruits do not appear to be affected. I've had some catfacing and some early BER which has since cleared up.