I'm seeing a couple of small spots there, but the uniform yellowing is not fungal blight (either "early" or "late"). I wouldn't expect overwatering if you're only watering 2-3 times per week, but you might check the soil to see whether it's soggy a few inches down. Or dry, though the most common sign of underwatering is wilting shoot tips.

This sort of thing is normal for a few leaves near the bottom, as they get shaded out and drop off naturally. More than the bottom quarter, I start to wonder what's wrong... if the top is growing vigorously, it's not a nitrogen deficiency. The next thing to consider is Fusarium wilt, especially if it's affecting one side or one branch of the plant rather than the whole thing at once. F wilt can cause that clear yellow color on bottom leaves, then march up the plant, and you're in a nice warm climate for it.

If it is Fusarium (more than a quarter of the plant doing this, the top is actively growing, and the soil moisture is okay)... unfortunately, there's no cure, and few solutions other than choosing more resistant varieties or growing in containers. Sometimes healthy plants will cope with it, but it tends to slow down growth on even the best, and weaker plants can die of it.

Hope that helps somewhat.

--Alison

This is senescent leaves with possibly just a tad of septoria and one lesion that looks like early blight. What you have is a shortage of nitrogen in the plant to mature fruit. The plants will be ok regardless but if you want to maintain production, they need some nitrogen.

When a plant does this, you have older mature leaves that are no longer producing like they should. The plant shuts them down and pulls nutrients back into the stem. This can be especially bad when the plant is stressed such as by a heavy load of maturing fruit.

DarJones

Just pull or cut the yellow leaves off. They are just taking up space and limiting air circulation. If the lesions spread then you should probably spray with a fungicide to keep the plants healthy enough for a season long harvest and good production.