I would definitely isolate it from your other plants, especially other phals. I agree with everything posted above me--even if it isn't diseased, a fungicide won't hurt it and repotting certainly won't. It's interesting that it's only the top most leaves affected. That's the sort of thing that makes me wonder if it could be environmental damage--any really hot or cold temperatures and/or drafts that might have caused it?

Cheers,
Rob