It sounds like the spores that they produce to reproduce. Without a pic it's hard to be sure, but I wouldn't worry about it.

Yes, does sound like reproductive sporangia. Are they on the underside of the fronds/leaves?

Hi Guest,

As both Beattie & Fractal say, this sounds like your fern is sporing, and it would be the more mature fronds only that would be bearing spores.

You say that the rusty substance is on the top of the fronds....do you mean the upper surface or the end of the frond?

Most ferns produce their spores on the underneath of their fronds but some - Osmunda, for instance - produce their sporangia as separate extensions at the tips of the fronds, or produce entirely separate structures (fertile fronds) held on their own stems.