Did a rotation of my plants today and noticed one with fan leaves between nodes 3-6 has this going on. Didn't notice it much with the intense lighting prior to this though I admit I was a bit lazy and didn't pay much attention to this one tucked in the back. Plant as a whole looks fine, tthe damage has a slight rusty color to it. Swapped 1kw MH to HPS and went to 12/12 a few days ago. Sorry for the HPS light, i'll take a better pic tomorrow evening, I tried my best with the white balance.