I'm thinking fertilizer burn. Browning leaf edges comes from that (as well as too little water). Not wise to use full strength MG on seedlings. I always use half strength. Different plants have different sensitivity to it.

is it coincidental that all the damaged leaves are the same size???

i think so ... it means the damage happen all in one moment ...

id just watch the newly emerging leaves.. to see if there is any current problem ...

ken

The newly emerging leaves have problems, too. If fertilizer burn and I just don't fertilize anymore will all be well? Or did I toast them? I still have time to scrap and start over with fresh seed if needed -- or just keep them well watered and hope the excess flushes out. Or would it be a good idea to transplant them into fresh potting soil -- certainly could do that this evening or tomorrow while I'm stuck in the house waiting for the monsoon to pass. ???

If fertilizer burn, you just need to water heavily, and leach the extra fertilizer out of the soil. The stuff is all water soluble. Everything will be fine going forward. After a week or so, half-strength fertilizer. Full strength fertilizer is dangerous in a container, because the soil at the bottom stays wet, and the fertilizer just builds up there. In the ground, the soil underneath wicks the stuff away.

I agree looks like fertilizer burn to me

Well, I decided to rinse off the roots in water and re-pot into fresh potting mix with no fertilizer added -- gun shy about soaking the containers to wash out the salts because I tend to have algae problems with my seedlings (although taking them outside now would probably address that issue nicely). Anyway, hopefully that will solve the problem, should know in a few days or so when new leaves start to emerge.