All I can see is the bottom of your plant. Are the tops full of green leaves? I don't worry about yellow leaves at the bottom of the plant. I think if it makes you feel better pulling off the yellow leaves will not hurt. I doubt those leaves are doing anything for the plant. If it keeps raining I'm expecting diseases though. A fungicide might help. People in the humid states use daconil at least that's what I've seen mentioned on the forums. I think with most fungicides you have to start before the problem gets bad. I wouldn't worry if you have healthy leaves at the top.

Marilyn,

I agree with Helen. If it is a fungal disease, Daconil would be the standard chemical fungicide. It works by coating the leaves and thereby keeping the fungal spores from attaching themselves to the leaves. If you are going to use Daconil or another similar fungicide, you really need to use it regularly from the first day the plants go into the ground. Once the plants have a fungal disease, Daconil is not as effective, though it may keep the disease from spreading to leaves that aren't yet infected. If you want an organic fungicide, Serenade or GreenCure would be options, or neem oil.

I cannot see the brown spots on the leaves clearly enough, but if those brown spots have brown concentric rings, that is a fungal disease called Early Blight.

The best way to prevent fungal diseases is to mulch the plants from the first day they are put into the ground in order to keep water from splashing from the ground up onto the leaves, carrying disease with it. As mulch breaks down over the course of the summer, add more. Also, water with drip irrigation or by hand-held hose, and apply water to the ground, not the leaves of the plants. When it is raining a lot, there's not much you can do to keep that moisture off the plant foliage.

This late in the season, I'd just remove the damaged leaves and probably not worry about the rest of the plant. You could remove the damaged leaves and spray the rest of the plant with a fungicide labeled for use on vegetables like the ones mentioned above.

I'm going to link the TAMU Tomato Problem Solver below and you can compare your plants to the photos of leaf issues there to see if you think it is Early Blight (or something else).

It is very hard to diagnose from photos because often plants have multiple issues occurring at one time, giving you so many symptoms that it is hard to figure out exactly what is wrong.

If you haven't fed the plants, you might try feeding them with a good water-soluble fertilizer (either synthetic or organic) to help them put out new foliage to replace the leaves you're removing because of disease. Often, plants that start looking bad this late in the season can outgrow the disease as autumn approaches, cooler weather arrives, etc. and they aren't as stressed, and feeding them can help them put out that new burst of growth. If you can get the plants to September relatively intact and healthy, they'll often set a lot of new fruit that you'll be able to harvest in the fall.

Dawn

Here is a link that might be useful: TAMU Tomato Problem Solver