I started some apple seeds (yea I know not true to type, I don't care)
This is what I did:

- stratified in fridge for 3 weeks (ziploc bag with soil)
- potted them (they were already starting to sprout) under indoor grow light
- they grew OK for 1 month (true leaves were formed), then all 20 of them damped off suddenly, within 3 day window. I was watering from bottom

What went wrong? I can think of a couple of things:

1. - high fungal growth due to discrete temperature rise - in nature apple seeds experience gradually increasing bacterial/fungal counts, as the season warms into spring. In my case it went fridge (38F) to 75F room temperature overnight.

This may have caused bacterial/fungal explosions which the plants immune systems weren't designed for

2. - grow light spectrum gaps - in the past, I had a grow light which couldn't produce true leaves. Tomatoes and watermelons never grew true leaves under it. Once I put them outside, they did fine.

This is a different light, but I bought a cheap plastic spectroscope, and both this one and the past one have a small gap in the light blue part of the spectrum.

What it can't be:
- soil. This is my backyard's soil and everything else grows fine in it as long as it's planted inground
- lack of light. It was very bright and I monitored the light duration