Could be a mag deficiency.
thanks anyone elseIt looks like a P deficiency. I had the same problem on some of my young plants, thought it was nute burn at first but as the dark greyish spots got bigger, I added more of my phosphorous rich nutes and the problem was resolved. Especially in flowering, it's going to be eating up that P fast!
nope i dont really stumped on this oneAre you using an ozone generator by chance?
It almost looks like ozone damage, if it is not in fact a nutrient deficiency.
If this is the case, way to fix it would be to move it farther from the plants.
don't suppose it's overwatering or a pH problem? I would have acually guessed at K deficiency, but people already said that...