- Any drainage in that container?

- What did you fill the container with? Soil? Potting mix? What?

- What temperatures have the tomato been exposed to?

- What kind of light?
- And for how many hours a day?

I have drainage holes, it's filled with peat moss and perlite, the temp is consistently around 70-73 Fahrenheit, and it sits under 2 4ft long 6500k 40 watt shop fluorescents.

I planted another rutger at the same time with the same potting mix plus worm castings and its doing much better, but the leaves on that one is also beginning to yellow.

Dave

When growing in a cup, you really don't need drainage holes because you shouldn't be watering enough for it to run out the bottom. Just give a small amount of water each day. Just enough to puddle slightly around the stem when you add the water. That's basically a quick tip of water and that's it.

Don't water for two days and then just give it a little drink daily.

Unlike container plants grown outside exposed to greater heat , very few indoor-grown plants, if any, require daily watering unless they are rootbound in the container. In that case they need to be transplanted to a larger container.

Add to that the well-documented fact that frequent, small amounts of water only encourages shallow rooted plants. I'd suggest some further research into this topic.

Dave

Most people will give too much water so it probably is not a great idea for the average person who will overwater..

It will work fine for a person that gives just the small amount of water that I described. Everyone will do things their own way. I'll keep doing what works for me and others can do as they see fit.

I deep water and allow the surface to dry quite a bit before watering, which is why I'm ruling out over watering.

That is your choice of course but surface drying is not a valid indication of the need for water. Stick your finger knuckle deep into the soil.

Over-watering is the singular most common cause of problems with young seedlings hands down. Nothing else even comes close. And fading/yellowing of the leaves is a classic symptom.

No one ever thinks they are over-watering yet as soon as they force themselves to cut back on the watering (not to mention the over-feeding) the plants improve. You have nothing to lose and much to gain by at least trying it.

Good luck.

Dave

A little seedling like that just doesn't need much water. It also doesn't need any fertilizer so you that could be part of the problem as well.

When you water, just aim for giving it enough to stay alive and nothing more.

I've never seen blight on a seedling.

I think a good potting mix with some extra perlite in it might have the proper pH and a little fertility. I am no expert but I have found that drainage is critical. I think you have a combination of potting mix holding too much water and a lack of nutrients.

You have a control because your plant with worm castings looks better. Do you have any more worm castings? I am thinking you could put them in water and strain it then use the 'castings' tea next time your plant is dry.

I am having a problem with some of my plants and the difference is I ran out of one batch of potting mix and used another on the later plants. I treated them the same but the earliest potting mix drained well and those plants look better.