Hi,
Mildew is a problem if roses don't get ample air circulation to help dry the foliage. If mildew is stil a problem there are several effective fungicides such as Phaltan, and Funginex that do a good job of preventing further infection. They need to be put on in advance of the problem as they don't do a good job of curing what is already there only protect the leaves from etting mildew.