What's wrong with the leaf?

Have you been spraying the plant or wetting the leaves?? New growth is ok, don't worry too much, it is not a disease. Quite often, damage like this is from the leaves resting on the soil at night when the lights are out and the plant lowers its leaves. It is 100% NOT septoria, which is almost impossible to get, unless you dig up soil from an infected tobacco farm and spread it around your pots. Always wait for multiple answers when you post a question and never go with the first random answer.........

Looks to me like a bit of nutrient burn or PH drift on a little seedling. Its not happening on the new growth and that's good, it has stopped in its progression. I would like to think the worst is past and all is fine but monitor for the next week or so to be sure. I do think it is done doing w/e it was, and the old leaves will not repair but the plant will grow.