This past year I obtained a large quantity of pure American chestnut seeds. I cold stratified them in the fridge and potted them up back in the spring. The germination rate was impressive and I got a large percentage of them to sprout. Since then, things have turned sour it seems. The little trees appear to have struggled all summer long in spite of copious rainfall. Additionally, I have many other types of trees growing in the same medium (Mel's mix) and every other tree is doing well. There is no discernable pattern from what I can see.

The ph of the soil appears to be neutral.

Can anyone offer any insight or advice?

From the looks of the leaves: too much water. If you've gotten a LOT of rain, you should remove the mulch and let the dirt breath; maybe root rot going on.

drainage is most likely your issue......in the wild american chestnuts often grow on rocky slopes.

We've had 10.75" so far in the month of July and in June (normally our second driest month) we had 6.5".

I had intended to plant them out this winter on a sandy hillside. Would transplanting them now be contraindicated?

NO - plant away. They'll be much happier in the dirt and that would also allow the water buildup to get released. You may want to consider removing then rinsing the dirt away from the roots (carefully) as the odds are good there's mold on the root stock. Make sure there's some organic material and leaves incorporated into the sand. They'll be fine and in Mississippi, the weather will stay plenty warm for the roots to heal and get acclimated/settled.

I went ahead and planted out four yesterday. Now I only have about 140 more to go.

In August!

Thank y'all for your advice. I'll follow up and let everyone know how it turns out.

Did you add some mycorrhizae? They would probably be happier if they had some fungal friends feeding them nutrients. Any type of bolete would be worth trying, there have been plenty popping up this rainy year. My best hunting ground is at the local mall, where they overwater and overmulch the oak trees in their acres of free parking. After a good rain, I can easily fill up a 5 gallon bucket. Just blend them up with some water and give your chestnuts a root soak to get them well inoculated. Then dump some more inoculate in the transplant hole.

We have lots of boletes popping up here as well. Since there is no way for me to plant out 150+ seedlings out in the near term, perhaps your idea will help me buy some time.

I went out today to begin clearing up the spot intended to be their permanent home and have developed a rather severe case of bilateral tennis elbow and golfer's elbow. I will be functioning at far less than 100% for the next three weeks at a minimum. Plan B is to put at many of the seedlings as possible on a large trailer and to park them under our carport during rains and then to water them by hand in a controlled fashion.

Anyone want to take a stab at the optimum watering schedule for a chestnut in a three-gallon pot?

Your finger is your friend. I wouldn't go to the trouble of steadily moving a trailer back and forth - the most important thing is extremely good drainage and leave the pots open to the air, not surrounded with plastics. If you stick your finger in a 3G pot about an inch and it's dry - water them. If you finger comes out dirty leave it be.
after the first frost, pack the pots together surround them with straw bales and spread a LITTLE straw over the pots to protect the roots. During the colder months, unless there's been no rain, snow etc for ten days, just gently spray the straw topping.

Get the poor things in the ground. Chestnuts have a huge tap root and do not do very well in pots. Kind of like a paw paw, much happier in open ground.

The first four we planted out a couple days ago looked no worse for the move as of this morning. So, we followed those four up with 20 more of the sadder looking ones today. As I was taking them out of the grow bags, the problem became obvious: the dirt in the top half of the bag was fairly dry; the dirt in the bottom half of the bag was so saturated that when squeezed, water would flow out from between my fingers.