Answer: The leaves of your amaryllis can turn a rusty brown color if they are not getting adequate light, but it is very unusual for them to develop the fungal disease called rust. See if you can wipe the rust spores off on a dampened paper towel. If so, use a fungicide manufactured for houseplants. If the orange or rust color does not wipe off, it is leaf damage due to poor lighting. Increase the light and the plant won't develop any more rusty brown color.
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