Check on the underside of the leaf for bugs. Looks sort of like something sucking life out. Anyway I get something like this on some of the older bottom leaves. The rest of the plant seems ok so not sure. Adding some info on your media and nutrition program will help ppl in diagnosing the problem. 😉

Could be the start of a calcium deficiency I need to take a look at the whole plant also to make a definitive determination! I don’t know if you’re still in Veg or in the middle of flowering with just the picture of the brown spots starting and not enough information in the question?

I would prob rule out a cal def. That is usually smaller speckled orange spots. Rust is a fungus and is airborne. If it is a fungus you could increase air circulation to maybe keep it from landing on other plants.