I don't have too much experience making my own transplants from seed, but let me propose to you that maybe, if you bought these seeds, they came with a disease. Technically, seeds for sale are indeed SUPPOSED to be "disease-free", but that doesn't mean it's always the case. I've had seedlings show symptoms of disease grown from purchased seeds, in fresh soil from those bags you buy. I've read similar stories on the internet.

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Here is a link that might be useful: {{gwi:59562}}