The problem is at the base of the plant. You could try piling soil up the stem and forcing new roots to form above that brown area.

Linda

Look up Fusarium Crown Rot. I had that happen once on my tomato plants; it looks like the bottom of the stem turns to wood, nothing you can do but get rid of the plant if that's the case.

How big are the pots? What mix are you using? Assuming adequate drainage. Not familiar with the fertilizer, is it organic? Sorry for all the questions. All that said, the plant doesn't look too bad. I'd remove the affected foliage for sure. And yea, the brown stem doesn't look too nice, but if the problem isn't too serious, maybe fill the pot almost to the brim with some additional mix. There's some room there.

Those plants are outgrowing their pots. Fusarium crown rot is a possibility, but unlikely if they are growing in commercial (probably sterilized) container soil. Many tomato varieties are fusarium resistant, and I think at least Yellow Pear (a heirloom) has some such resistance.