Take a big deep breath. It`s not rust, that gives you, well, rust colored spots on the leaves. A lot of things cause this.
Nothing else is going on? Haven`t moved it or changed soils? Then insects most likely.
So, I like insecticidal soap my self. Isolate the plant, give it a good soaking, everywhere, all the leaves, the stem, the soil, the pot. I put them in the shade to avoid burning leaves. I usually do this for a couple of days then let the plant rest, keeping it out of full sun and checking it for further signs of bugs, treat again as necessary. Move gradually back to full sun.
You can cut off that leaf if you need to. Mine have been through mealy bugs, scale, whiteflies, aphids, snails....you name it and they keep on going.
Tally HO!