Something appears to have damaged some of the roots and/or the circulatory system on that one plant. You can see in the interveinal cholosis in the leaves how some nutrients are getting through but not nearly enough. It could be something as simple as nitrogen burn of the roots as you are fertilizing heavily or something like like salt damage from the over-fertilization (see pic).

Even nematodes, soil dwelling pests, or more serious as with one of the wilts from contaminated seed. I can't say.

I would remove all the affected and yellowing branches and cut back on the fertilizers to see if that helps. If not I'd dig carefully around the plant to see if any soil dwellers turn up.

Dave

Thank you, Dave. It seems that one or two secondary stems coming off the main stem are more affected than others. I've clipped off many of the yellowing leaves, but the ones that remain on the affected stems continue to yellow and eventually shrivel up. The stems themselves are actually yellowing as well. If I remove those entire stems all the way back to the main trunk, would that help the rest of the plant cope with whatever is causing the root/circulatory damage?

Pjg, are you talking about "Leaf Branches" ? . If "yes" then cutting off the whole branch should be fine. Sometime I trim part of a leaf branch sometimes the whole branch. In the case of yellowing leafs, all the leaves on that leaf branch will expire shortly after.

Yellowing lower/older leaves (IMO) is a form of abortion by the plant to conserve energy/resources. That is why the new growth has the first priority. So the nutrients are there in the pipeline, going up to the new growth, which is further away from the root system and pipeline. .
It is possible that the root or part of it is damaged and not able to produce sufficient amount of nutrients. Or there isn't simply enough nutrients . In these cases the plant would prioritize its resources. Just My Opinion.

Yeah - remove the whole branch back to about 1/4" out from the main stem. The terminology with tomatoes is sort of confusing as that whole "branch of leaflets" is actually the plant's "leaf". So usually when someone says remove the plant leaf they mean the whole branch.

Dave

Thanks Seysonn and Dave for the feedback.

Didn't mean to imply that terms were an issue. Don't worry about it. But anything yellow I'd remove to reduce stress on the plant. Any green fruit on them might still ripen on the counter. Or you can always make fried green tomatoes. :)

Dave