Answer: You didn't describe the symptoms so I don't know if you are dealing with black spot or powdery mildew; both are common diseases of roses.
Black spot, a fungal disease, is the most serious disease of roses. Circular black spots with a frayed margin on the upper leaf surface are characteristic. As the spots enlarge or increase in number, infected leaves turn yellow resulting in premature defoliation. If the disease is not controlled, defoliation continues, which weakens the plant and reduces flower production. Such weakened plants become much more susceptible to other diseases and to winter injury. The fungus spreads from leaf to leaf during wet periods and new spots develop in 5 to 10 days.
Black spot must be controlled to grow good roses. Black spot cannot be adequately controlled without a good spray program. A complete uniform spray deposit on both sides of leaves is necessary. Spray applications must begin as new growth starts in the spring and continued at 7-10 day intervals and after heavy rains for the entire growing season. Do not let the disease build up before startinq a spray program. If the disease occurs, immediately remove infected leaves from the rose garden as they appear and rake up and/or discard old fallen leaves during winter months. At some time during the winter months, remove all leaves from the plants and discard or compost. In fall prune hybrid tea roses to about 18 inches and destroy the prunings. In spring prune again to about 10-18 inches and destroy the prunings.
Powdery mildew is a very common fungus disease. The characteristic white mold can occur on the surface of young leaves, shoots and flower buds. The disease causes leaf distortion but less leaf drop than black spot. Powdery mildew is usually more severe in shady areas and during cool periods. The fungus is windborne and can increase during periods of heavy dew. Remove and destroy diseased foliage and canes during the growing season and follow a rigorous fungicide program.
Excellent control of black spot and powdery mildew can be obtained by spraying chlorothalonil (Daconil) and alternating with one of the following three fungicides: triforine (Funginex), propiconazole (Banner Maxx), or myclonbutinol (Systhane, Eagle, or Immunox). Spray applications should be made every 7-10 days. Spray thoroughly to completely cover all plant surfaces. Follow all directions and precautions provided by the manufacturer on the label.
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