I asked a similar question in the spring of 2001, except that I was
asking "where did the whiteflies come from"? In my case, I started
seeds in brand-new APS kits, filled with brand-new seedling starter mix
(both purchased from a long-time favorite catalogue vendor, as well as
items I had used successfully in the past).
While I realize that whiteflies (or whatever) could simply have migrated
from my houseplants to the seedling trays, I offer this:
A) I have only 2 houseplants, neither of which showed any infestation.
B) Seedling trays were on a separate level of the house.
C) Door to room with seedling trays was kept closed at all times (to
keep pets from digging up, eating, or otherwise "insulting" seedlings).
The problem you describe sounds like "damping off" -- not sure what it
means, exactly, but it's the reason that most seed-starting kits advise
watering seedlings from the bottom.
So, this is really intriguing. I recently potted some seeds in some soil
here at home. The soil was brand new from Home Depot, potting soil, the
containers were clean. In some of those containers, there is a network of
fuzzy white fibrils all across the surface of the soil. What on earth? Is
this lousy soil, or is it a NC thing, or what? I've seen the
mold-on-clay-pots business, or bits of fungus near the edges of a pot, but
this is very pervasive & I'm somewhat hesitant to remove the plastic wrap
from the tops of the pots/jars etc (plastic wrap being to create the
greenhouse effect... err... you get the idea).