Hard to tell from photos but my best guess is salt injury. Avoid adding fertilizer to starting media.

I'd agree with salt injury or nitrogen burn. What seed starting mix specifically? What additives?

Dave

Thanks guys - Starting mix is Black Gold brand - contains sphagnum peat, perlite, domolite lime and Yucca wetting agent - been using it for years - no other additives. strange

So if you didn't add anything else - no fertilizer of any kind - then any chance it was an old bag or maybe a bag that had been stored badly got wet and turned anaerobic? Sometimes I have gotten hold of a warehouse carried over bag that had an off smell when opened.

Dave

could be - i did have an old bag that had been in the seed shed for at least a year - will try a new bag and see if there's any difference. Thanks for the suggestions, will post results as they come in.

JGN