I've read about using compost tea also, but haven't tried it. Some of posters on the rose forums say that by choosing your rose varieties for resistance to foliage diseases, allowing them plenty of free flowing air and sun, feeding them organics, and keeping them well watered is the best preventive. They feel the roses develop their own resistance if not sprayed. The good bacteria, fungus, etc. overcome the disease causing microbes. That's the idea behind using compost tea, but it's thought that it will happen on it's own also.

Maybe ask your question on the Antique Rose Forum and see if anyone there is using compost tea. Let us know what you find out.