From a posting on the Roses Forum by Jean Harrison of Nashville, TN
with her permission

The picture below shows a fairly advanced case of botrytis. In the early stages, the rose will have pink spots on the petals. As the disease progresses, the rose will brown and begin to rot as is shown in the picture below. In advanced stages, the bloom will be brown or gray and will be covered in a greyish fuzz. Left unchecked, botrytis will infect the canes and can cause canker.   

Botrytis is a common fungal infection and anywhere there is moisture, botrytis can lurk. In the dead of summer, this tends to be  less of a problem here (it's hot and dry usually), but wet, warm conditions will allow the fungus to thrive.    Contact fungicides work the best at controlling botrytis. Mancozeb and Daconil both work well. Systemic fungicides will help control the disease, but will not stop it once it is started. In the organic realm, sulfur works fairly well.  The Cornell method* has not been effective in my garden in dealing with botrytis.  Lime sulfur used as a dormant spray over the winter is highly effective in killing off over-wintering spores that can lead to canker in the spring.

*The Cornell method is an organic control for fungal diseases. Essentially, it is a mixture of horticultural oil and baking soda.