Have you googled this rust thing? I don't think it's mg def because that usually begins as golden yellow spotting in between the leaf veins. I'm wondering if you've given too much of a fert, but since it's been two weeks since the last dose, it's really hard to tell what's going on here. The damage appears random and brown, like heat damage from lights that are too close, but at 400w at 12-18 inches, I don't think that's it either. It's a tough one. I just did a web search and it looks like fungus can cause brown spotting with leaf curl, which is what you have going on there.
polishpollack Reviewed by polishpollack on . Mg deficiency or Rust fungi!! Alright lets start by saying i started noticing this on one of my plants about 2 weeks ago. At the time i was give a full dose of FF Grow Big and Big Bloom. I water with R/O water and everything is always buffered to 6.5-6.8 pH. Runoff is 6.2-6.5 pH. Last weekend I up potted everything to 3 gal pots using FFOF with 20% perlite added. Tuesday i noticed that it had moved from just the one plant now to almost all. At first i thought it was a Mg def. so I added Cal-Mag 3 days ago, but now Rating: 5