I have heard that using sodium bicarb - that trusty old bakin soda - or a
milk + water solution seem to work in rose problems. I'm trying it now so I
can't vouch for its effectiveness.

Apart from using the usual Yates anti-black spot fixes, you might also
consider using lime sulphate in Winter time - spray the mix *underneath* the
plant - that is - on the ground! It helps to kill all the nasty bacteria and
diseases that accumulated around the year. Suppose to be good in getting rid
of rust.