It happens one cell at a time, and very quickly. Yesterday they appeared healthy, today they’re down. Other cells within the same tray are seemingly healthy.

I have to guess damping off. Soil is too moist. The fungus that causes it spreads effectively, so everything in the same cell will be affected. Damping off is wholly a disease of seedlings.

https://kaybeebio.com/blog/damping-off-onion-crop-protection/

https://extension.colostate.edu/docs/pubs/crops/02940.pdf

One way to avoid damping off is by sterilizing the soil before planting. I bake mine. Of course, if the plants are left outdoors, the soil will likely get reinfected just by wind-blown fungus. So sterilized soil is really only effective indoors.

Thank you. Is there anything I can do to prevent it from happening to the other cells that are already growing?

Allegedly, soil treatment with dilute hydrogen peroxide will kill the offending fungus. I've used a 1/3 dilution of 3% H2O2 to prevent root rot in other plants. Just water with that. Now, that's pretty strong. Might want to try something a little weaker. Again, I don't do it for damping off because I use sterilized soil. Maybe someone can be more specific about dilution for onion seedlings.

Your onions are to tall! and the plants can't maintain the turgor pressure (water pressure) to hold them up. To much vegetation and not enough dirt and water so cut them back. YES, cut off up a third of the onion tops and don't be shy. Yes it's recommended and all nurseries top onion seedling, and I do it multiple times before planting mine in the ground.

Well, that doesn't account for why individual cells are suffering, and why the fall-over happens so fast. If that were the reason, all cells would be affected equally and they'd tip over gradually.

Not necessarily , each cell might have different moisture content so that might account for one cell acting different to others. Are you certain plants are dead or is it possible weather was cold enough to make them do that? I've planted onions in the Fall that looked awful after extended cold then recoup and do fine going into Spring. Onions transplant very well so I'd suggest transplanting into new soil instead of trying to sterilize existing soil.

It's damping off. Has nothing to do with them being too tall (not that that's really a thing). If you look close you can see the threadlike stems indicative of damping off at the soil level.

Rodney

Looks like dampening off to me too. Those seedlings look waisted at the soil line. If the pots are in water, move them to a dry tray. If they are moist but not wet, take them out of the tray and let them bottom dry a bit- the tray walls can help trap a lot of moisture. If you have little/no air flow, set up a fan to get some surface air blowing.

I don't think bottom drying is going to fix much. The fungus is there and established,. When you replant, the fungus will go back into action, though keeping the soil drier may slow it a bit. Look for the stem getting shriveled right at the soil line. As noted, I see that in your picture, Damping off is an issue for many seedlings, in which the stems have not firmed up enough to repel the fungus.

From what Theforgotenone said I took another look and enlarged the picture and would say you're all right. All I saw was tall plants and for me they will usually fall if not trimmed. For over a decade of sprouting my own seeds I've encountered damping off only on a few occasions with peppers and some plants in the mustard family, but never onions.

I use capillary mats for watering, and the soil is always at the maximum water retaining capacity it can hold. so I don't think it's a watering issue or all my plants including the onions would be trashed every year. In fact all my plants sit on wicking mats until I transplant them to a bigger pot or in the ground outside.

As noted in the references I linked to, damping off is a big problem for onions in both home and commercial plantings. See also this piece by Bayer. https://www.vegetables.bayer.com/za/en-za/resources/disease-guides/onions/damping.html

They make the point that onions are most susceptible to damping off at low light levels and high soil moisture in the flag leaf stage. That's where you have ONE stem popping up, as what we're looking at here. I should add that overfertilization just induces more fungal growth, so it's best to NOT fertilize seedlings until they are more mature. Strong lighting and good ventilation will help dry the soil surface, which tends to prevent it as well.

Are these bunching onions and if not shouldn't they have already been transplanted to allow more space for each plant? As a general question for the group, can onions be as successful in pots as raised bed/ ground? I've grown identical plants (never onions) in ground and in pots and those in ground always had less disease problems and out performed overall. I've credited my lack luster success with pots to inability to judge correct moisture and nutrients so that raises the question of should newbies start with inground or raised beds.

Hard to tell from the picture, but the general rule for transplanting onion seedlings is when they're 5-6 inches tall. Crowding is no issue up to then. You seed thickly, and then pull the plants apart for transplanting, Works well for me.