Answer: You are right - this is a fungus disease called rust. Some antirrhinum claim to be resistant, but these also succumb to rust from time to time and cannot be relied on. The rust only affects antirrhinum so other plants in your garden are safe, but any other antirrhinum may get infected, so it is best to discard infected plants and avoid growing antirrhinum in the same place next year.
If you treat early, at the first signs of infection, a fungicide such as Systhane Fungus Fighter or Scotts Fungus Clear Ultra, is likely to be give good results, but once the infection is widespread it is too late.
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