Yep, that's the stuff, though it's usually seen on the undersides of the leaves, with the tops of the leaves looking brown and blotchy. Here's a photo I found for comparison:
http://www.dudak.baka.com/nf27.html
It can absolutely come in on the mix. It thrives in areas where there's decomposing organic matter. Not a good sign that the mix was being reused--sorry Traci.
Here's a link on how to deal with rust on roses--they have a remedy I've never heard of before but might be worth a try before you pull out the big guns. It involves spraying with baking soda and water, which sounds harmless enough, except for the salt that's going to end up in the medium if you do.... http://www.ehow.com/how_9356_control-rust-roses.html
Personally, I would go with a fungicide like John was saying, and just have done with the problem. Make sure you hit all your plants.





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