The following is a good article on black spot on roses from Rutgers University
Black spot, caused by the fungus Diplocarpon rosae (the imperfect stage of this fungus is Marssonina rosae) is the most commonly recognized disease of roses. The disease can cause damage all season long in temperate climates where leaf tissue remains wet for extended periods. Plants chronically affected by black spot become unthrifty and are prone to winter injury.
Black spot is most troublesome early in the growing season. In the spring, fungal spores are produced and disseminated from lesions on canes and leaves infected the previous year. These spores infect young leaves (6- to 14-days old) when a 7-hour period of continuous leaf wetness occurs. Disease development is greatest at temperatures of 75 to 85oF with greater than 85% relative humidity. Symptoms appear as black leaf spots (0.1- to 0.5-inch) on the upper leaf surface within 3 to 16 days following infection. These spots have feathery edges and are accompanied by yellow “halos” of leaf tissue. Spores produced in these spots continue to infect newly expanding leaves and canes throughout the summer. Black spots may also form on the lower leaf surface about a month following infection. On first-year canes, irregular, raised, red-purple blotches appear that become blackened and blistered. Diplocarpon produces a toxin that causes affected leaves to turn yellow and defoliate prematurely. Indeed, heavily infected plants lose much of their carbohydrate reserves, and as mentioned above, grow poorly as a result and become more susceptible to winter injury and other stresses.
Since the fungus overwinters on diseased leaves and canes, rake old leaves and prune diseased and damaged canes before spring. Avoid overhead watering and excessive shade, and when planting, space plants to avoid excessive humidity. Disease management before the growing season begins is critical; control is difficult once black spot is established in a planting. Resistance to black spot varies among the different types of roses. Floribunda, shrub, and climbing roses tend to be more tolerant to this disease, whereas hybrid tea, grandiflora, and miniature roses are more susceptible.
For best results, spray fungicides after budbreak (mid-May) and repeat at intervals specified on the label. Compounds labeled for black spot control include Armada, Bacillus subtilis, calcium polysulfide (dormant) captan, chlorothalonil, Concert (outdoors only), ConSyst, copper (Badge, hydroxide, metallic, salts, sulfate), ferbam, Junction, mancozeb, maneb, myclobutanil, neem oil, paraffinic oil, propiconazole (outdoor use only), Spectro, Sporan, sulfur (dusting, elemental, flowable, wettable), SysStar, thiophanate-methyl, trifloxystrobin, ziram, and Zyban. Use a surfactant to enhance fungicide coverage if this practice is listed on the fungicide label, and rotate classes of chemicals to reduce the likelihood that fungal resistance to compounds will develop. Pay close attention to spray practices during wet periods, especially when caring for roses that are highly susceptible roses to this disease.
Black spot on rose leaves with feathery purple/black spots having no definite margin. Photo by Paul Bachi, University of Kentucky Research & Education Center, Bugwood.org
Information from Ann B. Gould, Ph.D., Specialist in Plant Pathology, Rutgers University. See the full article including other rose diseases at http://njaes.rutgers.edu/pubs/plantandpestadvisory/2009/ln082009.pdf