Answer: Mold on the growing medium indicates overly wet conditions. You can avoid the problem by watering your trays or pots from the bottom so moisture is wicked up through the soil, and by increasing air circulation around the seedlings. (This can be accomplished by thinning so there's a fingers-width between each seedling.) Place the trays or pots where fresh air will freely circulate, but keep your seedlings out of cold drafts.
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