What disease is this?

Overwatering and/or stress during sprouting or early growth stages.

i would guess radiation stress, looking at your grows and info but maybe you can tell more about how you run those three lights and location of plant could be a hot spot i see you run high room temp have you messured foilage with laser temp? could be a few degress warmer and bring you over 30. Remember these are still young plants.

This caused by overwatering your plant, if you let it recover and dry out before watering again, she'll be fine. Hopefully, I answered your question and remember " Happy Growing and Free your mind one puff at a time ".

One of my LSD-25s did the same thing on a couple of the leaves and eventually worked itself out, the next set that came out were free of it and it never came back again. I'd monitor it, but wouldn't worry unless the plant starts to suffer or slow growth.