The first suggestions were that it might be a problem with too high a pH, or an iron deficiency.
On Mar 13, I ran out to the local hardware store and got a cheapo soil test kit to do a pH test on the soil. As I expected, based on the peat moss in the mix, this is an acidic soil.
On Mar 14 I visited a larger garden store in Ann Arbor and picked up a bottle of Ferti-lome brand iron supplement.
I mixed the Ferti-Lome with distilled water in the recommended 1 Tsp/half gallon ratio, and poured about 1/4 cup of the solution into each seedling's pot. On Mar 16 I diluted the full strength solution with an equal amount of water and repeated, and on Mar 17, gave the plants about an 1/8 cup of the 1-tsp/half-gallon mix.
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