I've lost two seedlings to rot this year, and these were seedlings that were grown in black plastic pots, in the sun, for a few years now. One of them was gone before I could save it; the second one was subjected to various drastic measures (including bleach and H2O2) and I thought that I had saved the last fan (out of about 6), but just discovered yesterday that it was gone, too (this, after being repotted in a new pot, with new soil, in the shade).
To top it all off, today I noticed a loss of a fan (to what looks like rot) on a different seedling (also black plastic pot, also grown in such for a few years). What the heck?!!
If I could grow all of these seedlings in the ground, I would. But some, of necessity, have to go into pots. (Now I'm afraid to dig some possible volunteer seedlings, because if I pot them... will they rot, too?)