Rust is quite unusual on ferns. More than likely what you are looking at is reproductive spore. google fern spore pictures and look at the varieties.

My reply on your other post. http://forums.gardenweb.com/forums/load/pests/msg090234278070.html?1 Beyond that, if you're concerned instead about the small brown spots on the backs of the leaves, understand that those are groups of spores (reproductive units) which help grow new ferns. Here is a link that might be useful: your original post

I have fragrant mini roses; yellow or rose.

It should, but ferns are really slow when they've been cut back. Try sprinkling cayenne pepper all around it and see if that stops whatever is eating the plant. Unless you know what the rodent is, then you can repel accordingly.