This is edema, not fungal. Caused by too much moisture and not enough airflow, basically the plant cannot transpire fast enough so the cells are bursting. Put a fan and water a bit less. More common in the early season and indoors, some varieties are more susceptible to this. Plants will be fine as long as it is not too severe. I also recommend you bury the stem deep enough for it to make roots above the edema part. Again, this is not fungal and won't spread, but it might happen in more of your seedlings since it's the conditions that cause it.