Leaf rusting, drying

Can't really tell, just one well-shielded leaf showing signes? The plant might be shedding that leaf - if it doesn't spread nothing to worry about. Plant doesn't look overfed, but the fact it's all 3-5 fingers says it is experience some sort of stress or imbalance in feed. if you've done a bunch of LST lately, probably why, but one more thing to consider. also, quite hard to assess leaf symptoms under blurple lighting... get some normal white light in there when trying to diagnose. leaf symptoms are not discrete so you need more info to rule out possibilities. If it's isolated i would not react right now... 1 leaf being shed is not a problem. I don't defoliate plants according to bro-science nonsense... mid-to-late flower my plants sometimes shed a handful of leaves despite being as healthy as you can expect, otherwise. It's normal.

Without an image, it's almost impossible to help you. A diary also helps a lot. More info equals more accurate help. It can be from deficiency, toxicity, messy feeding or an infection.

Its nutrient burn First tought i advise you to flush the plants and take of rusty leaves and see if It persist