Answer: Blackspot causes premature leaf drop and left unchecked, may defoliate the plant. Continued attack results in unattractive plant foliage as well as reduced size, number, and quality of blooms. Successive seasons of defoliation may weaken the plant and increase susceptibility to other disorders. Black spot cannot be adequately controlled without a good spray program. You'll find a variety of sprays at your local garden center to help control black spot. 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. 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.
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