MVIsland, the diagnosis of your problem was INTERVENAL Chlorosis ... lacey yellowing between the veins of the leaves ... caused by micronutrient deficiencis, usually iron or magnesium. That is why I recommended Ironite or Fertilome iron supplement. Also, as Mulio said, a good Miracle Gro type tomato food should contain all those micronutrients, but I recommended the Ironite + Epsoms salts as a booster. What Elskunkito describes sounds more like classic nitrogen deficiency caused or compounded by chronic wet root ball causing rotted roots which then cannot take up sufficient nitrogen and phosphorus (he noted some stiff purple stems along with the gradually yellowing leaves). Without pictures, I cannot confirm the diagnosis, just the same as I needed pictures of your plants to confirm micronutrient deficiency witnessed by intervenal chlorosis. However, what elskunkito describes, and he describes it quite well, SOUNDS LIKE gradual and general chlorosis moving from the lower and outer extremes upward through the plant. Why is this nitrogen deficiency (most likely)? Because nitrogen is mobile in the plant. That is, the plant is capable of moving nitrogen from where it is least needed (the cotyledons and lower, older leaves) and transporting it to where it is most needed (the growing tips). Nitrogen is essential for photosynthesis, cell division, new growth and starch (sugar) production. Why is there no nitrogen left in the old potting soil? Because nitrogen is volatile and evaporates into the atmosphere or is washed out of the potting mix by irrigation. Nitrogen must be replenished into the soil in regular dosings for proper plant health. Lastly, folks who continually repeat the old saw that seedlings do not require fertilizer are doing no one any favors. Stop it please. This instance with elskunkito's excellent description of symptoms as well as MVIsland's previous problem two weeks ago are just 2 recent instances that clearly prove growing tomato plants require proper nutrient feedings. In elskunkito's case, he clearly stated his plants are 8 - 10 inches tall with minimum 3 sets of true leaves ... and "a couple months old." THEY NEED PLANT FOOD!!! My caution to you all is to ignore these folks who repeatedly say tomato seedlings do not need plant food. They are only repeating bad, incomplete, uneducated information they have gleened from other uninformed or improperly informed people at other websites, IMO.

I would start with a 1/4 to 1/2 strength solution of your organic seaweed. They look nutrient deficient to me. When I do the initial transplant, assuming I am using a straight, basic potting mix without any fert in it, I start feeding a weak solution of fish/kelp, or seaweed fert, or Miracle Gro regular water soluble fert. I am attaching some images of plants with nutrient deficient issues. Susan Here is a link that might be useful: nutritional deficiences in tomatos

I think the most likely culprit is epinasty due to overwatering and potentially a soil mix that is too heavy (too much soil/compost in containers can compact and/or get sludgy). The general rule of thumb for watering tomatoes, whether in-ground or in containers, is to dig down into the soil first to see whether it's dry a couple inches down (deeper for in-ground plants) and actually needs water. If the soil surface is still moist, watering is absolutely not needed and could be detrimental. There's also a possibility that the mulch or compost or "farm dirt" could have been contaminated with herbicides, but herbicide injury is usually more distorted looking than that and usually is accompanied by yellowing or other discoloration. You can test this out by planting peas in the soil next to the tomatoes. If the peas germinate and emerge looking normal, herbicide is not the problem. That doesn't look like any of the common viruses to me.

Thanks for your input! I potted the 3" seedlings (with one or two sets of true leaves) into Solo cups (and a few yogurt containers) with soil up to the leaves. So the stems were buried deeper when I transplanted. I put the roots right at the bottom of the cup (with a little soil under) and then filled up to the leaves. Some have room left to add more soil as they grow (most are maybe 1/2 to 2/3 full of soil). They are not touching the cold windows - they are in a sunny south window, a few inches from the glass. I have some other ones that are at least 12-18" away from the window and they have same symptoms. It's at least a month before they can go outside. I have been plucking off the dead foliage and am definitely hoping they'll grow out of it (and that it's not disease that's going to spread!). The top soil looks dry, but they are watered regularly from the bottom. I water when the cups feel light, usually every 2nd day or so. I've added a couple more pics showing the whole plants.