Can you scratch the rust off the leaves? If so, its a fungus. Spray with some Defender or some Enzyme Komplete, or whatever with sulfur in it.
If you cant scratch it from the leaves, then its or a deficiency, or a burn from a foliar you did few days ago.
If you're in soil, flush the plant with water until you get a good 30% run-off. Take a look at the ppm of it. Apply some B1 in conjunction with some
Cal-Mag by foliar under the leaves. Give them a good 500ppm of AB at the next watering.
If you're in coco, Flush the plant with a good 400ppm of AB until you get a 30% run-off. Check out the ppm of run-off. Apply some B1 in conjunction with some
Cal-Mag by foliar under the leaves.
If you have a hand of some Green or some Spray and Grow or some PowerPack or whatever contains some micro and oligo elements, add it to the foliar spray in both cases.
It should be back healthy in few days.