I had the same problem, and I assumed it was calcium deficiency after I gave my Ph the all clear (using r/o water). I fixed my problem by getting sea-shells from the ocean for what if I recall correctly is calcium carbonate. Ground some up, sprinkled it on top of the soil, and even went as far as to grind some up and put them in water, and then water the plant. All of my rust spots have disappeared. I would do some more research and make absolutely sure that is the problem though. No two plant problems are the same. Thoroughly diagnose your plants before you do anything to the soil, especially check your Ph, makes it much easier than trying to correct wrecked soil.