Answer: Knock Out roses are among the most disease free roses you can plant. I wonder if the leaves are yellowing from overwatering and are developing black spots as a secondary infection? The symptoms of black spot begin with circular black spots, frequently with fringed margins. Yellowing and defoliation are common in susceptible cultivars. In wet weather, spots may become very severe and run together, making large irregular spots. Control practices include avoiding dense plantings (good air circulation all around the plant is important). Avoid overhead watering. Rake up and remove all leaves at the end of the season. You can try removing the yellowing leaves as soon as you notice them, but I suspect the canes might also be infected. A copper-based fungicide should stop the disease in its tracks. Ortho's Rose Pride is a fungicide and it should stop the disease. But, since it's early in the season, you might try pruning your plants to remove any suspect wood and foliage. The new growth that develops should be disease free. You can use the fungicide to maintain the health of the new growth and keep it disease free. If you use a preventative spray in April and May of next year, your roses should not develop black spot next year. The red chip mulch should have no effect on the health of your roses. Best wishes with your roses. Best wishes with your roses.
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