Rose Black Spot 

"..... has probably done more to discourage potential gardeners of  roses than any other factor. Left untreated, it can  completely defoliate a them by midsummer.  Focus on prevention by beginning a treatment program early in the season.

This fungal disease is caused by the pathogen Marssonina rosae or its sexual stage which goes by the name of Diplocarpon rosae. It is a two cell spore that is not visible without the aid of a microscope. This disease is characterized by black irregular lesions appearing to be on the leaf surface, however occuring under the surface of the leaf. The lower leaves are almost always the first to be infected unless the cultivar is in close proximity to a structure such as the wall of a house or a trellis, in which case the upper leaves can show first infection. These spores over-winter in infected leaves, canes, or on rough surfaces. They do not over-winter in the soil.  Splashing water, resulting from rainfall or sprinklers, generally provides a means of transportation for the spore, however it is thought that some spores can hitch a ride on the back of a sticky insect."* 

Rose Flora provides the many benefits associated with Mity Gro including  balanced soil, better nutrient availablity and improved overall plant health.  The reprint on Organic Gardening/Farming provides an excellent overview on how to achieve a balanced soil and the associated benefits.  You can have a copy of this article sent to you automatically at your request.

Application instructions are available at Application Notes.

 * excerpted from article by Norman Rose in the 1998 Annual of the ARS.