Some battery acid stains "acid burn" look almost identical to fertilizer staining or rust. You can the difference in battery acid though because it will have a white center from concentrated sulfuric battery acid. So, rust has a red center, battery acid has a white center. Both look the same but do have distinct differences. Be careful of many commercial rust removers as they will just eat the cement which causes etching and permanent damage.
These are before/afters of fertilizer staining. You can tell the difference between fertilizer stains and battery acid burn because fertilizer stains will normally have a very concentrated center (red). Fertilizer stains may also have a tail if they are on a down-slope or may just be small orange dots on concrete if they are on flat concrete. Some commercial rust removers are good at removing fertilizer staining but are not so good at removing the deeply embedded "tails."