I read recently that if you dissolve two un-coated baby aspirin in water - use a spray bottle - that the mix is excellent for a variety of fungus. Its hard to tell from your picture what it is. I'd take the plant to a garden center where they can get a better look at it.
It came on shortly after I moved my geraniums outside to get some sun so I brought them back in and it stopped getting worse but is still there. I really do hope that they won't die as I have put so much work into them.