I’d suggest looking for a bacterial fungicides. They’re least likely to hurt the seedlings. You can usually find them under the name “biofungicide”.
If you’re planning to start more trays, it might be worth looking at mycorrhizal inoculants. Those are just fungi that help the plants instead of hurting them, but they can also out-compete the bad fungi if given a head start.
Thanks! I have several pouches of mycorrhizal inoculants. I’ll try a light dusting.