Question: What is wrong with my Roses?
Answer: Roses are a popular plant known for their large, elegant flowers. Many gardeners try their hands at growing roses in their gardens and landscapes. Poor disease control is one of the most common causes of failure with roses. Black spot and powdery mildew are important diseases to be aware of when growing roses.
Black spot is the most serious disease of roses in North Carolina. Black spot is a fungal disease that causes circular black spots with a frayed margin on the upper leaf surface. These black spots start small and will enlarge and increase in number. Eventually the infected leaf will turn yellow resulting in premature defoliation. If no action is taken to control the disease, defoliation will continue which will weaken the plant and reduce flower production. These weakened plants will become much more susceptible to other diseases and to winter injury. Black spot spreads from leaf to leaf during wet periods and new spots develop in 5 to 10 days.
Black spot has to be controlled in order to grow good roses. Black spot cannot be adequately controlled without a good spray program. Do not allow the disease to build up before initiating a spray program. A complete uniform spray on both sides of leaves is necessary. Spray applications must begin as new growth starts in the spring and continued at 7 to 10 day intervals and after heavy rains for the entire growing season. 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 the winter months. During the winter, remove all leaves from the plants and discard. In the fall prune hybrid tea roses to about 18 inches and destroy the cuttings. In the spring, prune again to about 10 to 18 inches and destroy the cuttings.
Powdery mildew is a very common fungal disease on roses. Powdery mildew is evident by the white mold that occurs of the surface of young leaves, shoots, and flower buds. The disease causes leaf distortion but less leaf drop than black spot causes. 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. The diseased foliage and canes should be removed and destroyed during the growing season along with following a fungicide spray program.
There are several fungicides that can provide excellent control of black spot and powdery mildew. A recommended spray program includes the use of the following active ingredients: chlorothalonil and alternating with one of the following three fungicides: triforine, propiconazole, or myclobutanil. Spray applications should be made every 7 to 10 days. Remember when spraying to completely cover all plant surfaces for best results. Read and follow all label directions when using fungicides. Overall, it is important to be aware that black spot and powdery mildew are common diseases that will occur on roses unless a proper spray program and good plant care is practiced.
Reminder: A growing program this year is the “Sampson County Friends of Horticulture.” This program offers monthly “How To” Horticultural Seminars. Please call (910) 592-7161 for more information. Please call the Sampson County Cooperative Extension Center at (910) 592-7161 with your horticultural questions and to register for any upcoming events. Be sure to check out the Ask An Expert Widget at sampson.ces.ncsu.edu for any questions you may have