Very doubtful it's disease, or it's neighbors in the growroom would likely be affected, too. What does the underside of the leaf look like?

To me, it looks like one of two things.

1) Have you used a magnifying glass to look for bugs on the undersides? (I doubt this is the cause, but I'd check anyway)

2) It looks like the larger injuries are possibly from leaf-tip irritation. When a fan blows the leaves (leaftips, for example) back-n-forth against another leaf, it can irritate the leaf being rubbed, it will cause a sore that looks similar to that, which will brown-up as the damaged part dies. Old injuries don't "heal", but new growth shouldn't be affected.

The browning of the smaller leaves kinda looks like a transplant injury or some other mishandling. Keep an eye on 'em and let us know if anything get's worse.