It looks like both to me. My eye's not as well trained as Bob's but I think he's on to something about TRSV or something along those lines, at least. Frenching is more of a symptom than a cause, I think, with many possible contributing factors.
EDIT: If it is/was affecting some plants in cells and not others, a virus sounds like a very likely underlying cause. Frenching is otherwise mostly down to poor soil conditions which would be hard to replicate in cells unless you were literally trying to do so.
I ran out of planting soil and used some of my daughters planting soil that smelled like septic after a few watering. I dumped the water out of the trays several times due to smell.