Booklady - I have just seen a couple of commercial organic products in my garden centre here in the suburbs of London, but I dont know how widely available they are for you. Anyway, the first is called 'Bio Bizz Leaf Coat' and is made of natural latex - this is supposed to strengthen plants against harmful leaf fungus and mould spores.
The second, which is probably much more helpful to you (if you can get it), as it is specifically for controlling fungal diseases like powdery mildew and black spot on roses
, is called 'Vitax Organic 2 in 1 Spray'. It also kills most of the common bugs, mites and fires that love snacking on your tender rose
shoots. This one is a spray-on, made of a blend of natural plant and fish oils.
Iv'e never used either of the above myself so I cant vouch for their effectiveness, but the chap beind the counter seems to think they do work. I did buy the Vitax stuff though, just to see for myself, as my roses
are also showing signs of blackspot (it cost me £4, which is around $7 i think).