Rust spots

red rusty spots can be Mg deficiency. do those start pale? could be Ca, too, or lockout from something that is too high in relative concentrations. more info is need to be certain. you can fill in gaps with knowledge of recent behaviour and guarnateed analysis data on each of your nutrients' labels. if the ppm's don't point to something else, epsome sale (magnesium sulfate) is fine to use and cheap. Knowing the ppm's of your feed at dillution you mix is very helpful when you see leaf symptoms. pH, concentration of nutrients as well as ratios of those concentrations are all equally important. Always do the easy stuff first.. check pH, if you have an EC meter that's awesome too. Just knowing what you feed is enough most of the time without any meters (well, pH is huge, but soil helps avoid problems).